Used to work (IT, game/web development) 40h max a week. And could barely handle it. As I grow older (33) the tolerance is going down. My body asks for more exercise and less sitting (or standing doesn't matter) hours. So 40h a week is too much now. I'm at 25-30h tops. So I try to make the most with that time. Of course I must say that I have a condition, since I was a kid, that makes me feel terrible if I sit for too long (more than normal). Circulation or something. My pressure goes way down I guess when I'm concentrated (going to the movies was hell). So I learned to be effective and do a lot in less time. I now only work at home. Thank god. My goal is to work 3, 4 hours a day tops.