A cat's heat is between three and 14 days long, but most cats are in heat for about a week. If the cat does not mate during her heat, she may come back into heat in only a matter of days.
Cats typically have their first heat cycle at about six months of age, although that varies between individuals. Once they come into heat, they continue to cycle unless they become pregnant. According to Dr. Janet Tobiassen Crosby, cats that are not used for breeding should be spayed before their first heat in order to prevent disease and accidental litters.