The usual time for visible heat signs in a dog is 2-4 weeks. Even if the dog has no vaginal bleeding, it can be in heat. Dogs usually ovulate after the vaginal bleeding stage and that is when they can actually conceive puppies. Non-spayed female dogs will go into ‘heat’ or estrus usually twice a year. The age at which they start their cycles and the duration of the cycle varies greatly between the breed and individual dog. The small breeds can show signs of heat by 6 months of age, and the larger breeds are usually older when they show signs of heat, from 6-15 months of age, typically.