The female cat comes into heat for 2-3 weeks several times a year. The cycle continues for several heat cycles or until she is bred. They then go back into season while weaning the kittens, but should not be bred during this period. According to experts, a reasonable rule of thumb between breeding is '3 months from the birth date of the kittens, plus one month per kitten born before rebreeding'.
Sometimes after three weeks or so a cat will shows symptoms similar to being in heat even though she has been bred - it can be confusing.