Dogs typically bleed during the first half of their heat cycle, known as the proestrus phase, which usually lasts about 7 to 10 days. During this time, the bleeding is most noticeable and is often blood-tinged vaginal discharge. The amount of bleeding varies by dog, with larger dogs generally bleeding more than smaller ones, and some dogs bleeding very little due to grooming habits

. The entire heat cycle lasts about 2 to 4 weeks, encompassing four stages: proestrus (bleeding phase), estrus (mating phase, where bleeding lessens and stops), diestrus, and anestrus. After the bleeding stops, the dog may still be fertile for several days

. In summary:

• Bleeding duration: approximately 7 to 10 days (proestrus phase)
• Total heat cycle length: about 2 to 4 weeks
• Bleeding lessens and stops during estrus phase, which lasts about 5 to 10 days
• Some dogs may have spotting or very light discharge beyond the main bleeding phase

This bleeding is a natural sign that the dog's body is preparing for potential mating and fertility.