Your dog can have heat cycle up to 3-4 weeks. Heat cycle in dogs can start at the average age of 6-8 months and can be repeated twice in a year. This heat period begins with swelling of vulva and vaginal bleeding that can last for 7-10 days. After the cessation of bleeding, mating takes place and if she becomes pregnant then pregnancy can last for 63-65 days. After the delivery of puppies, she can come in to heat again after 4 months.