Most female dogs, or bitches, bleed for 18 to 24 days while they are in heat. Some bitches have their first heat as early as six months old. Healthy female dogs go into heat twice a year.
In general, females from smaller dog breeds go into heat earlier than large-breed dogs. Some large bitches do not experience a heat until they are two years old. As dogs age, the duration of the bleeding phase often becomes longer. Some dogs experience bleeding for more than a month. There is no age when a dog naturally stops the heat cycle. The only way to stop a bitch from going into heat is through surgical spaying.