It depends upon how many stars they have at their best position and how many they have in the position you're trying to change them to. I've found that 2-3 games for each half star is a pretty sound estimate.

For an offensive lineman to become a wide receiver for instance I'd consider that quick.

But having said that, this is part of the game I love the most that players are not pigeon holed into one position and can with on field experience change positions.

Also while we are on the subject is a (Bpos) 1.5 star player better than a 2 to 2.5 star player who " can also play adequately as "?