Estrus in female cats normally first occurs between 5 - 10 months of age, with the average age around 6 months. The female cat has 2 - 4 heat cycles every year, each lasting about 15 -22 days. If she is bred, the heat cycle seldom lasts more than 4 days. If successful mating does not occur, estrus may last for 7 -10 days and recur at 15 - 21 day intervals. It is possible for an unmated female to cycle every 3 - 4 weeks indefinitely. Cats also have a heat cycle 1 -6 weeks after giving birth, so a female may be nursing one litter while pregnant with another. Cats do not have any vaginal discharge or swelling of the genitals during their heat cycle, as seen in dogs. Behavioral changes are they only obvious signs. A cat in heat carries her tail to one side, keeps her hindquarters elevated, exhibits "treading" movements of the hind legs, and seems unusually affectionate. She spends a good deal of the time rolling on the floor and seems very restless. The cat's voice seems more piercing than usual and she may "call" for 1 - 2 days before she accepts the male.
A lot of females will display inappropriate urination while in estrus. Some even spray like male cats. My own queen pees on my bed when she is in heat so when I get up in the morning, I make my bed and then cover the top of it with incontenence pads to prevent the mattress from being ruined.