Every six months, for well checks. And of course if they become ill or injured beyond what I can handle on my own.
Every six months for a few reasons. So my vet is familiar with them, and what they look like healthy, she'll be more likely to notice something amiss that I, not being a trained vet, might miss. So they are more familiar with the process, and it isn't as stressful as it might be if they only went once every year or less. So if something (such as a dental problem) happens to develop shortly AFTER a vet appointment, the cat will not have to suffer with a bad tooth (or whatever) for almost a whole year before it is noticed.
Cats are notorious for hiding pain, including things like dental problems.
So, since I can afford the small appointment fee twice a year per cat, I take them every six months.