A female dog typically stays in heat for about 2 to 4 weeks, with the average duration around 18 days. This period is known as the estrus cycle or "heat" and can last anywhere from 14 to 21 days depending on the individual dog and breed

. The heat cycle has several phases:

• Proestrus : The beginning phase lasting about 7 to 10 days, characterized by vulva swelling and blood-tinged discharge. The female is not receptive to males during this phase

• Estrus : The actual "heat" phase when the female is fertile and receptive to mating, lasting on average 9 days but can range from 3 to 21 days

• Diestrus : Follows estrus, lasting about two months, where the body either supports pregnancy or returns to rest

• Anestrus : A resting phase lasting 3 to 5 months before the cycle begins again

Signs of heat include vulva swelling, bleeding (which may last about a week to 10 days), increased urination, and behavioral changes such as increased nervousness or receptiveness to male dogs

. Female dogs usually go into heat about twice a year (every 6 months), though this can vary with breed and size. Smaller breeds may cycle more frequently, while larger breeds may have longer intervals between cycles

. In summary, a female dog’s heat lasts roughly 2 to 4 weeks, with bleeding typically occurring in the first week to 10 days, and fertility peaking during the estrus phase within this period