You will get lots of different answers on this.
For me it depends on the prospect. If he is dominating a level and has ML potential I move him up.
If he is dominating a level and is a sure fire no doubt about it ML prospect I may move him up 2 levels in a season.
The only place I usually leave a sure fire ML prospect for a full season is AAA.
I have a certain number of AB's. Starts, IP for RP that I want at each level before moving a guy up. It is fewer at the Rookie, LA, HA levels than at AA and AAA.