WebMD states that female cats go into heat several times during the year unless they are being bred. The heat cycle is influenced by the seasons, and is activated when there are 12 hours of daylight. Other factors include the temperature of the environment and the presence of other cats.
Cats that live in the Northern Hemisphere have a mating season that lasts from March to September, while the mating season of cats that live in the Southern Hemisphere lasts from October to March. Female cats that have reached breeding age often go through multiple heat cycles that last approximately 14 to 21 days in the early spring. Heat cycles occur farther apart in late spring. The heat cycle continues until the cat becomes pregnant or until there are less than 12 hours of daylight each day.