I wouldn't breed them because there are enough staffies available...but to answer your question I would think at 2 years old , this will give you enough time I think, to research how to breed and do it right. Read the thread about what to look for in a good breeder, this is what you should live by.
You need to make sure health is top notch, by getting a vet specialist to certify your dog is free of any defects or any genetic diseases, also a joint/hip certification is done, all this can't be done usually until the dog is full grown which is at 2 years, (in dog years they are roughly 21 yrs old), also the temperment of the dog is free of aggression or/and timidity, fear, they aren't hyperactive and such this will be passed on to the pups if the adults are that way, but even if the these pups aren't that way, genetics sometimes skip a generation and they may still get it, also when you breed you want to better the breed so make sure your dog is what is needed to better the breed, do they have titles? of any kind, champion puller? obedience trials or search and rescue or anything? And when you do find a mate..this criteria should be met by the other half of the genes going into the pup. Don't be a back yard breeder, make sure you have the money to get all this done, a good breeder never makes any money, they barely break even, my Aunt breeds and she has been doing it for 40 years. When she sells a pup she even makes sure they put a $100 deposit on a puppy training class, and when proof is shown you get your deposit back. Never breed because you want one just like your dog, there is no guarantee this will happen, don't breed for color, these are all excuses given by backyard breeders.
I didn't want to preach but I see what happens when breeders pop out pups with no thinking of any of this and that's why bully breeds are getting a bad rap. IMHO every pup born and sold puts another to death at the shelter.