
Guest,
It is believed by responsible breeders who care for their dogs that it is wise to wait until around the 3rd season or 2 years of age. However, please note that it is obviously possible for a bitch to get pregnant on her first season, but not wise as they are not physically or mentally mature and may have problems giving birth or rearing a healthy litter, resulting in large vet bills and possibly no puppies.
If your bitch is KC registered, the KC will not register puppies born from a bitch who was mated before she was 12 months of age, so she would have to be at least 14 months of age before giving birth.
As with all breeds, Staffies have recommended health checks which should be completed to ensure that puppy buyers are not open to take the breeder to court for negligence. Someone who knows your breed will no doubt point these out.
Also, you should check that your bitch is of a good enough standard to breed from, or you may find it very hard to sell puppies and may have lots of unsold dogs eating you out of house and home, quite literally. :D
Hope that helps.
M.