Should I start a stud service and when should a male pit bull stop mating?

I have three beautiful REAL American Pit bull Terrier males. They are registered with the UKC and ADBA. Two are 2 years and 1 month and the other one is 18 months. I constantly get there shots and take them to visit the vet. I feed them high quality food and give them Nu-vet supplements. Once I see if they're healthy enough to breed, should I allow them to mate. I only want responsible dog owners to let their female mate and I want to see paperwork stating they're healthy enough to breed. Could someone inform me more about this. Its really not to make money but I would like to know how much money I could lose. Making money isn't really important; I just want to see more REAL APBT, but if it will hurt me financially then its not worth it and I can get them neutered. Also, when should a male dog stop mating? THANKS!!!!!