Cats are induced ovulators and are never really out of heat :/. They can always get pregnant if a male forces himself on her---the act of mating is what stimulates ovulation. As for actively calling, she'll do that for about a week, then keep doing it every couple of weeks until she is mated or is spayed (and some cats will take a break in early winter).
It would be best to spay her. Being in heat all the time is very hard on a cat's body, and her calling all the time can be very hard on your mental health! If you need help finding a spay clinic let us know.
Sandwich is in heat right now and very vocal hissing growling at the other cat and their was white stuff coming out of her is that normal I just happened to catch it before she cleaned herself