Although the heat cycle varies from breed to breed and in individual dogs, the average time a female dog is attractive to male dogs (or in heat) is 18 days. Most females are attracted to males for less time than that.
During the first part of the estrus or heat cycle, a female dog has vaginal swelling and bleeding. She gives off scents that make her attractive to male dogs but is not yet interested in mating. When the female dog ovulates, usually (but not always) around the 10th day of estrus, she is fertile and highly interested in mating with any male dogs in the area. The heat cycle usually happens twice a year for mature female dogs.