it is preferable to avoid playing a player too much, but there are no *permanent* effects of playing time.
Well, Coco's right as far as GS goes. Too much playing time (>80 min in a week) will hurt a player's game shape, but that's not permanent.
The other side of that question, though, is how playing time effects experience. It's possible that getting lots of minutes (especially in league games), is actually
good for young players since it will help build up their experience.