From what I've heard, ball pythons never stop growing, just slow down. Typically though, they get to about 4 ft at 2 years of age, and that's considered mature.
But, if you feed him 2 mice a week for his entire life, he will be underfed after about 1 year of age. You should feed them a prey item that's no bigger than the biggest part of the snakes body about once a week.
I've always followed the rule of food being no larger than the largest part of the body. I consider this a fact for the colubrids and most snakes, but I find that with the Ball Python, I go by the size of the head. I feed no larger than twice the size of the snake's head. Some females can be very broad and I'm not sure they should eat prey that is as large as their body. I'm not saying I'm right, it's just that's what I do.