Canine menstrual cycles vary in their longevity; size is one factor. On average, a dog's entire menstrual cycle lasts about a half-year to two-thirds of a year, but extremes are four months and almost two years. For most of the time, a female dog's in anestrus, the resting stage of her menstrual cycle. For a period of a week to three weeks of the cycle, she's in heat.