Once a feral...?

Short answer: it really depends upon the personality/genetic make up/DNA of the cat. It can be done, but it can take four weeks to four years. It is rewarding, and it changed my life.
Long answer: My friend rescued four kittens, approximately 4 to 6 months old, the day after Christmas 2010 after a 20 inch snow storm. They and the mother cat were abandoned at my condo building when the in-humane person moved. She socialized them; I helped at little. Prior to this time I'd never been around cats. Taz and Ritz became socialized relatively quickly, in about a month. Ripley and Riley took almost six months. I adopted Ritz and she is the love of my life. She learned how trust, I learned how to be trustworthy.
I take care of a feral/stray/abandoned cat colony in front of my condo building. In January 2011 my friend and I moved four (eventually five total) cats into a spare room my friend converted to a Cattery. The purpose was to socialize them and adopt them out to Forever Homes. Two or three "turned" fairly quickly; we have placed four cats into Forever Homes. We have been working with "Mystique" for well over a year. Only recently have we been able to pet her briefly, and that is only because same friend introduced her two large DOGS to Mystique--love at first sight. She absolutely adores the dogs, headbuts them, lets the dogs groom her, and will even jump into bed with my friend just to be near the dogs. Mystique is still a 'work in progress'; prior to the introduction of the dogs we thought we'd have to return her to the colony.

It really depends on the cat, they're all different. Are you sure it's always been a feral, for instance? Some feral cats can tame quickly, others take ages, others don't tame at all. Even ones from the same litter/background will act differently to people. I think usually a once truly feral (no prior human contact) will always remain at least a little skittish, unless tamed at a very young age.

Blaze was caught around 6 months... he didn't move off the couch for a few weeks, was very nervous for the first year, always has hid from strangers, and afraid of garbage bags and garbage truck noises. Also, while this may not be related, it likely is, he sprays around the house and occasionally attacks at random. Past that, he's an extremely loving boy, very vocal and even plays hide and seek!

Meanwhile, Blacky had prior human contact, she was spayed and had unreadable numbers in her ear, no idea on her age at the time we started feeding her, she had been on the streets for some time, she had poor teeth and wasn't a kitten. She was semi-feral (meaning she'd had previous contact with people)... but she'd reverted to a feral state and was afraid of people. She was outside, and with persistent effort while feeding her, she eventually let me get close enough to pet her a year later. It was like a switch was flipped; I hadn't hurt her on contact, and she allowed me to get somewhat close to her from then on. With more and more work, and slow progress, she eventually became a great extremely well adjusted - although independent - cat. She sleeps on my bed at night curled up on top of me, sleeps on my lap at the computer, follows us on walks... doesn't allow people outdoors to approach her unless it's us, but is not afraid of strangers. She is perfectly happy at the vets! And she isn't afraid of a vacuum unless it's in the room she's in, she'll just saunter out of the room. 10 years ago, just shutting her inside the house (once she allowed me to pick her up) caused her to freak out and jump out a broken window! She's come a long way, for the first few years she'd bite and hiss if we scratched her tummy but eventually she even allowed us to do that, it just took a lot of trust to build.

It is generally true just about all feral cats can learn to be domestic pets. It depends on how hard their life as a feral was whether they will ever be close and loving pets; but in my opinion ALL ferals have the potential at at least live amically with humans.
My Apache was a three month old feral when my dad trapped her and gave her to me. Now at 16 years old, she is my heartcat, rarely far from me, and has been since she was 18 months old. It takes much patience and time and I could not touch her for the first six months I had her without her slashing and biting me severely. Now here she is, in my lap constantly and asleep by my side.
Some severely abused cats will never trust humans, but can learn to live beside humans as friends on an individually accepted basis. A friend of mine has one cat that was kept in a very small cat carrier for over a year before being rescued by the ASPCA and rehomed. The little cat had a broken foreleg, tail broken in multiple places, broken ribs and two skull fractures. Apparently when the little cat was let out of its cage, the father of the house used to kick it across the room whenever it was close to him... Lugh is now a very timid cat when it comes to humans and trusts only my friend. During the day, no human lays eyes on him, but every night Lugh (pronounced Lew) creeps into Sue's room and into her bed where he sleeps with her every night, purring and happy. If he hears movement in the hallway, proof! he is gone and hiding in Sue's closet until the sounds outside her door go away.
Truely unmanagable ferals are actually few and far between and live in areas where people are not commonly in contact with the feral colonies. My one uncle is a dairy farmer and monitors a TNR colony that lives in the State forest lands next to his farm. The only people these cats see are the TNR devotees that come and trap the new cats for spay and neuter and they are and can be very viscious. They definitely are not pet material. But other places, where cats live close to humans, almost all those cats could be tamed and live with humans if someone put the time effort and money into doing it.
Last summer, my BFF and I rescued an abandoned pet off the streets. Her name is Rosie and she was kept in the house of her former owner until she became about six months old. Then the owner got tired of keeping up with the litter pan and starting letting Rose and her brother, Jesse, out to "do their business". The boro trapped Jesse and neutered him in their TNR program and marked him with a tipped ear. But Rosie did not frequent the TNR dry food station that attracted Jesse and so remained unspayed. So, of course, she got pregnant. I was sitting on my friend's porch and noticed this really, really skinny black tiger with cinnamon tortie markings in the year next door. I asked Kim about her and she told me it was Rosie, her neighbor's abandoned cat. I asked to borrow two cans of food and went over and offered Rosie some food. When Rosie was gulping down her food I noticed she was lactating and told Kim she had a litter of kittens and Rosie was starving to death. Kim immediately took over feeding rose several times a day and I agreed to take Rosie and whatever kittens she had in so I could tame them and find them homes. When the kittens turned about six weeks old, they started crawling out from under the garage where Rosie had her nest. Kim caught the babies, then bent down and picked up Rosie and put them in her downstairs bathroom. Kim kept the kittens and Rosie until the kittens weaned and then they came to my house and I set them up in the spare bedroom where I finished taming Rosie and started on socializing the two kittens. It is slow and it is very fulfilling watching the kittens go from skittish, scared babies to trusting and affectionate pets over the past year. Taking the three of them for spaying set back to trust needed for socialization for a while, but they have recovered. The problem is I am in love with them now and I cannot give them up
Hoist on my own petard!

And how's the former feral doing, bluemilk?

I should share a story too... I often tell the story of Hilda, whom I named so because she reminded me of my aunt Hilda, who constantly had something to object to everything. (Actually, the cat's full name is Hildegaard von B' now, B being our neighborhood name, and it's inspired by Hildegaard von Bingen, whose music I love and I passed by Bingen on a trip in the Rhine, which I loved too).

When I first moved into this building 4 years ago, Hilda wouldn't let anyone sleep at night, that's how ferocious and noisy she was. Neighbors feared her (I feared her too) because she could bite and scratch for no reason. I didn't have cats or know the first thing about cats back then, except that they are scary, grey things that jump out of the garbage room in buildings and make disposing of garbage a scary experience.

When I started feeding the strays downstairs at the superintendent's insistance, this cat started rubbing on me one day and I stopped her immediately, told her that was off-limits. "I give you cats food, but touching me is off-limits!"

Soon, however, she taught me that a plan is something you have until a cat decides otherwise. She continued rubbing on me, but wouldn't let me stroke her head for more than a second. She was the terror of the street, the poor colony cats preferred to go hungry than come eat the yummy food I set down for them each night, so I asked for help from someone who knows a lot about cats.

That's how Hilda got TNR'ed immediately and I was told that she was in heat at the time of the spaying, and that she was probably an abandoned cat who was simply terrified. I started giving her preferential treatment at feeding times, and when I adopted Princess off the park bench (2 strays fit on a bench for meals, and the ones who had claimed those 2 spaces for their own are nowadays house cats of mine, so there was room left), Hilda took her place, which gave her access to spending hours next to me, and one day, she timidly, tentatively made towards my lap.

I also trained her to respond to her name, especially when she got scared. She learned to respond to my call and come to me for refuge and reassurance instead of attacking when she feared a dog, cat, human. Nowadays she spends life between the porch of the building, my lap and frolicking around the garden, and when something makes her feel unsure, she runs to me and sits on my lap or hides between my legs. There hasn't been any noise, biting or scratching from her in about a year. She's a sweet lap cat with occasional feisty behaviors. She has a mommy, me, even though I can't take her upstairs to my home, and spends more time with me, playing, being brushed and petted, than my own house cats.

^
Myulchee has known me for about a year now, but it's been a turbulent year! Didn't meet her very often and it was a bit hot and cold: one day semi friendly and the next time really skittish. It was always like walking on eggshells around her. But there were signs of warmth, like when she'd follow me to the outside of the dorm and hang around outside. Or she'd follow me down the path to the dorm and from bench to bench, always staying one bench away.

And unfortunately after we trapped her, the vets treated her horribly and she just hides in a closet now all day long. We didn't help much as we tried to pet her and pushed her limits a bit too much, until she panicked and burrowed under the linoleum flooring, where she stays probably 18 hours a day or more, whenever we go onto her balcony especially.

So now we're wondering how long she'll play the hiding game...it's been two months now...
But it's great to hear success stories like yours and we're hoping Myulchee will be similar one day.

There's a stray who always sits on the other bench. She wants to be near me, but not that near, hehe. After about 16 months, she started occasionally sitting on the same bench as me, and even eating next to me sometimes. It took a very long time of heavy investment in building that trust, lots of yummy foods (which cats somehow interpret as you being good to them), strolls together in the area gardens, rescuing her from a storage room in the next-door building where she'd gotten trapped for days, loving on her sister, laying down hidden comfy pillows and old clothes for them to sleep on behind the bush in the winter, treats, talks, playing with Da Bird, seeing me being kind to all cats, seeing other cats let me touch them, etc.

A cat in hiding, I always think they're doing a lot more than hiding. They're assessing the place, the new territory, studying it, learning the map of what is and what isn't safe for them, and studying the people. Cats don't do anything gradually. They one day, out of the blue, when you least expect it, make a jump of faith. And usually it happens to me that they make it when I've kind of given up on gaining that trust. I think our relaxing has a positive influence on them. I just pay very close attention to every detail in their behavior and start doing more of what works and less of what doesn't work or hinders.

Lady Nikita, who came into my apt 4 months ago and won't leave, is only now learning to ask for what she wants. Sometimes we take things for granted, such as that a cat will make their needs/desires heard somehow, but strays (same as abandoned children) have to learn to ask. First they have to learn that if they ask, things will be given. This is simple to us who grew up in families, but not for those who always had only themselves to depend on. I reward Niki by each time she makes a sound to ask for something, trying very hard to discover what she wants and giving it to her. I think that when they learn to communicate with us, and internalize the cause-effect of asking for what they need, it's an important step in trust, as they feel more in control, more confident of their relationship with us. Things make more sense to them, they know more what to expect.

Prince very often will meow in a questioning, lamenting tone, because he learned during a long year of repetition, that I'll come to him immediately to ask what's wrong, I'll try to find what's bothering him, so he does it just when he's bored to get my attention for a few minutes and maybe a treat or something new and fun such as me emptying a top shelf in the closet for him to climb to.

Just like humans, in order to trust, cats need to see regular patterns, things that repeat always the same. Your cat, unfortunately, started trusting and then became confused and reticent because of the veterinary ordeal. It's not too late at all to start again because the cat knows very well that YOU didn't do her any harm at any stage. She's just confused, and I wonder if it's more the new living situation than the ordeal. Give her all the time in the world, she's better off hiding in your house than roaming the streets anyway, and do more of what you observe helps.