I usually make the decision based on future commitments to other salaries and what I can piece together for cheap in free agency.. Also if my team is destine to win 70-80 games no matter what I will usually leave the guy down.. If my team is destine to compete for 80-90 I will call them up early if its an upgrade.. If my team is destine to win 100+ I usually leave them down as long as possible.
If the player is a SP, I usually hold him back 20 games and overwork my other SP's for the 1st 20 games while limiting the prospects IP in the minors.. Not sure if that works the way I think it does.. But that's what I do.. If its a 1B, RF, LF or DH I usually can find a cheap FA to use until the prospect is 22-23 yrs old and I try to get max development in the minors.. I think it's poor managing if your plan is to hold a guy in the minors for 4-5 seasons and then wait 20 games no matter what..