The length of a dog's heat varies, but on average, it lasts between 12 and 21 days. Some dogs may have a heat cycle that lasts for as little as two days, while others may be in heat for up to four weeks.
Estrus is assumed to begin around the time the first external symptoms appear. The first visible sign is the swelling of the external vulva, but depending on the dog, this may not be obvious. The most commonly noticed sign is bloody discharge from the vulva. Dogs usually ovulate about seven to 10 days into their heat cycle, but this varies. In order to prevent accidental pregnancy, female dogs should be kept away from intact males during the entire heat cycle.