I try to allow for some rest period in my sessions to make sure I'm always alert and playing my best game. While a whole session may last 6-8 hours for me I try and allow for rest periods every couple of hours to keep me fresh.

As far as winnings per session you can't be concerned at that microscopic level. Even the best players can have major downswings while still playing good solid poker. That's why you hear the "One long session" response very often and few people will offer different advice. There really isn't any other advice to offer - play your best game and stay true to the "theories" and you should be a winning player over the long run regardless of a few losing sessions. We all have those days - or longer in some cases. Hang in there.