That's a great question - and the answer is that each girl is different. Unlike dogs which have an approximate 3 week cycle, cats have no schedule. Some girls come into season and are out within 4 or 5 days. Others stay in season for 4 or 5 weeks. The one thing they do all seem to have in common is that the cycles do become more frequent the longer they go without being bred. If you are not sure if your girl is in heat, the best thing to do is keep her away from males all together. Some cats also have "silent" heat cycles, where you have no clue that they are in season, and they suddenly end up pregnant. If you're not planning on breeding your cat, the best thing to do is just to schedule a spay when it's convenient for you and take her in. Trying to figure a spay schedule to alter a cat that is not in season can be extremely difficult.
Please let me know if I can help further.
Feline Healthcare
16 yrs health care mgmt & issues relating to cats, reproductive issues and multicat environments