If your female cat does not mate, she will go into oestrus as often as every two to three weeks, for several months each year, until she either mates or is spayed. Cats as young as 4 months old can go into the heat cycle, and they should be spayed as soon as possible unless they are pedigreed and you wish to breed from them. If spaying is not immediately possible then your vet may be able to give her hormones to stop the cycle. After all, the cycle is extremely uncomfortable for her, even if it does not inconvenience you. Your cat will be in heat for one or two days, but won't really come all the way out of it until you have her spayed.