The normal and most common is for the Dam to come into heat at 6 months of age and then every 6 months there after.
Some Dams come in at 4 months and some at 12 months for their first heat.
Some every 6 months and some every 5 to 8 months. All Dams vary, so you should keep track of what your dam is doing and when.
Most Breeders Breed the Dams on 3rd heat or after 1.5 or 2 years of age when proper genetic testing is complete and the Vet gives approval that Dam is an adult and ready.
Dog gestation period - A normal pregnancy lasts 63 days.