There is no hard and fast rule. You have to make that call on an individual basis and always remember, they are animals and they sometimes react badly. Just a point to ponder: for several years the number 1 dog bites came from golden retrievers, simply because they are so friendly people let their kids bounce all over them and sometimes they get hurt and react with a bite.
on a personal note, I had a rottweiler and we were out on a walk when a neighbors 2 pitbulls came running and surrounded us and started attacking my dog. I kicked at them and then the owner came running, picked them up by their tails and took them inside, no wonder they were mean. Several years later I had to have her put down because she had been so traumatized by those dogs that she had become more and more aggressive to other dogs. We tried various trainers suggestions and nothing worked. I loved that dog, but sometimes we have to make hard decisions. And for that reason, I do not like pitbulls at all. Nor do I trust them one iota.