Dogs typically bleed for about 7 to 10 days during the proestrus phase of their heat cycle. The entire heat cycle lasts around 2 to 4 weeks (14 to 21 days) on average. Bleeding usually starts with the vulva swelling and continues for roughly the first half of the cycle, with bleeding decreasing or stopping during the estrus phase when the dog is fertile and ready to mate. However, the length and amount of bleeding can vary depending on the dog's size, breed, and individual characteristics. Some dogs bleed heavily, while others may have very light spotting. If bleeding lasts significantly longer or is very heavy, it is advisable to consult a veterinarian.