you can breed from a bitch at anytime once they have had a season, however it is recommended to wait until they are 2 years old. In the USA I believe you can't register pups from a bitch under 3 years old. Two years gives you an opportunity to have all the geetic tests and hip scores carried out
The UK Kennel Club will not register a litter if the bitch was mated before she was twelve months old, although with most breeds it is usual to wait until at least her second season. Depends on the breed and how mature she is.
I would not breed from a large breed until she was at least two years old.