I've heard that you really should only be playing for 45 minutes at a time and a maximum of 3 hours a day to avoid tendenitus, a severely crippling wrist/arm condition that can be developed. Obviously these numbers will change depending on the individual, or they might be conservative... or they might not be right at all. I know I've gone far past 45 minutes of straight playing and haven't had that condition yet. But after playing for a while (especially classical guitar) I do notice a funny tension in my wrist (typically my right one), and this isn't good at all. What do you guys think about tendenitus, do you think it's a real concern and that you should watch how much play or not?