I moved this to the appropriate section for you so more people would respond.
Cats got into heat about every 3 weeks. The heat cycle lasts about 7 to 9 days. Indoor only cats will go into heat all year round due to the fact that there's constant light.
Unlike female humans and some other species, (we are spontaneous ovulators), cats (and rabbit) have induced ovulation, meaning that the ovulation is induced by mating by a neural reflex. So, unfertilized egg will be inside the female cat untill she mates. That's why female cats, especially indoor ones, go into heat every 14-21 days. Also the length of daylight influences the heating cycle; as the daylight length increases so does the heating cycle...
Frequent heating poses the danger of pyometra, infection of uterus, for the female cat.