Well, i use to play two tounaments of The Master League of the Intelipoker. So, when i do well in this tournaments, i use to play about 7 to 9 hours a day.
i usually aim for 4-6 tounaments a tournament day or 4-6 hours of cash games. It depends on my runs as well, if im doing pretty good and connecting a lot i usually play the full time however if im having some bad runs i would stop way earlier and give my self a break
My session begins with the opening of 5-6 tournaments and the addition of several more in 2-3 hours, and ends after the last tournament ends. Play more than a day is possible, but the result is sure you will grieve for a long period of time.
It all comes down to my mood and my schedule, it can vary from 2 hours up to even 10 hours if I'm playing huge tournaments.. I never grind for over 6 hours though as it will lead to me getting tired and performing rather poorly!