Female dogs may have bloody discharge during the proestrus stage of reproduction which lasts about 9 days. The next stage is estrus stage, or when a dog goes into heat. This phase may last an additional 12 days, for a total of 21 days of discharge.
Female dogs bleed at the proestrus stage, the first stage of the their cycle, and this lasts for about seven to ten days, while some say it’s 4-20 days. The average cycle for female dogs in heat occurs approximately once every six months.