ratings will grow according to playing time and the quality of the coaches. How fast you bring him up depends on your situation, if your guy is better then what you currently have in the big team and you are contending, then bring him up even if he only has 2 or 3 pro years, he will progress better because of the better coaches. But if your rebuilding, and he has less then 4 pro years and is not significantly better then what you currently have progress him normally in the minors. As long as your not tanking theres nothing wrong with that.