Baseball is a multi-level professional sport. That means some players are aa players, some are aaa players some are ML players. Obviously the vast majority are not ML players. With that in mind a player needs to demonstrate that he has mastered the current level to be promoted. That's a general rule. Top rated prospects are a little different, I will promote them with less productivity as long as they weren't a total failure at their current level. I typically have a 3-6 year plan for top prospects and promote them according to that plan as long as they don't bomb at current level. Typically they need to get to the majors by age 25.