Most small to medium-sized female dogs will come into heat around 6-8 months; large and giant breeds can be 12-16 months. Heat cycles last anywhere from 10-21 days and the timing and duration of fertility varies between dogs. Just because you always bred Old Bessie at 12 and 14 days, does not mean all dogs have the same cycle. We do not recommend breeding a female dog until after age 2 and after some basic genetic evaluations have been done. What these tests are depends on the breed, but might include tests for hip and elbow dysplasia, genetic bleeding disorders, genetic heart disorders, etc. Male dogs can breed any time after sexual maturity, which can be 8-18 months depending on the breed. We also recommend the same genetic testing for male dogs.