I have some questions about how long oit usually takes for 2 female kittys to get aolng. I have a 7month old and just yesterday adopted a 6 week old female. So far my 7 month old is hissing at mandu. how lond does it usally take for them to get along?
Well, for one thing, unless this 6 week old was rescued as an orphan or some other drastic scenario, that's too young to be adopted. They should be 8 weeks minimum, 10 or even 12 weeks is better.
But anyhoo, you got it, now here's what to do!
You really should separate them for a day or two with a door between them (the kitten could probably be okay in a bathroom... maybe a small guest bedroom or den/office area too). That way, they can sniff at each other under the door and play "pawsies" and get used to each other by smell alone.
Then you could either switch rooms for another couple days, OR (if you're really in a hurry), do supervised time together in one room a couple times a day.
Putting a blanket or something with the other cat's smell with the opposite cat should help too, during the "their own room" times.
This is the "official" way to introduce cats. I, however, do not have places to separate (studio apartment), and I just have to put new cats in the same room as my cats. So far I've been lucky and had no fighting...hissing and growling, and hiding wherever they can. Eventually they got used to each other and got along great. (well, Paizly always avoids everybody, but she's a cranky old woman!)
It took about a month for Paizly and Nebbie to really be calm around each other.
When I fostered some kittens, it only took a couple days... but then, Nebbie was happy to finally have somebody to play with! Since Paizly is boring
If your 7 month old is hissing at the new kitten, I recommend doing the full "introduction" phase that vivid dawn described. You should not leave a little kitten that young alone with an older hostile cat. Make sure that your kitten's space is warm and cozy with a box & blankets/towels (even close the vents if it is a cold room). Even if the cats were getting along already, you would still have to have a closed off space for the kitten whenever you aren't around because kittens that young tend to crawl into weird places and chew on things like electrical cords. I would keep her in there for 2ish weeks with gradual supervised meetings with your older kitten. Make sure they get lots of positive things when they are together like treats and their favorite toys.