My mother thinks they stay in heat til they get bred. If that is so, it explains the annoying behavior of my unfixed female cat (my other cat is a fixed male, so he isn’t helping matters).
I tried search engines but keep getting porn sites, so am turning here for assistance.
IIRC - about a week and a half
I found lots of hits searching for “cats veterinary”, rather than “cats in heat” 
Not a porno site in the bunch.
Anyway, go here. Your mom is basically right. Please consider getting her spayed - there is probably no shortage of unwanted kittens at your local shelter.
Thanks much for the link.
And this advice:
while appreciated, is unnecessary.
First of all, she hasn’t left my apartment in two years, which means that her getting preggers is highly unlikely.
Secondly, I can’t afford it.
Thirdly, she generally bugs the crap out of me and will soon become a barn cat, get pregnant, and give the farm more barn cats.
Plus my fixed cat beats up on her.
I’d still get her fixed. We got a female kitten, after having 2 fixed males. When she went into heat, she triggered our oldest male to begin spraying everything in the house. It didn’t stop until we had her spayed.
But the damage was done. When my wife became pregnant, her scent triggered him to start spraying again. And he’s still at it, 6 months after our daughter was born.
We’ve tried “friendly” pheromones, but they have limited success. One of the next steps is Valium, for the cat. (Although after he ruined my $120 sport coat, I coulda used one!)
Mine sprayed my straw purse, which wasn’t worth $120, but I really hated not finding out about it until I sat down in the corp. president’s office for my job interview.