I think everyone keeps telling you to go back to the standard BRM rules, because each day is different at a poker table. You don't know how variance will affect you, so a money goal isn't a good idea. Somedays you will lose, and somedays you will win, but if you've hit your target in 30 min, why stop if you think you can win more and you're still fresh? As far as a set amount of time, that one is better, but I've noticed my attention span and optimal playing time varies a lot depending on the day. With online poker, I start going brain dead after about 6 hours, but I can more sense that it's time to wrap up whatever I'm in and stop for the day. Sometimes, that only takes 2-3 hours to hit that point, and I don't want to be losing money when I know I'm playing my worst.