It really depend on you personality and endurance. If you can concentrate for a long period of time play for longer sessions, if your mind tends to wander after and hour take frequent breaks. You just have to be aware of how you feel (very aware) and have the discipline to take a break if you feel less sharp...even if you do want to continue playing.
Now I generally play 3-4 hours per night during the week with few if any breaks (I am usually involved in 2 MTTs or SnGs) on the weekend either Saturday or Sunday I get a single 8-9 hour session in and take a breaks at roughly 3 hour intervals. My breaks last from 15-30 minutes...and usually consists of throwing the ball/playing with the dogs - I tried jogging on the treadmill but I would mull over hands, and in the end it was not much of a break. You just have to know where your limits are...most days 3-4 hours straight is not difficult for me, there are days where my mind starts to wander after an hour - those are the days I just stop playing...because if I don't I will lose a bunch of money.
Do your chores, pay bills, spend some time with your wife/husband/girlfriend/boyfriend, get lifes little things out of the way so when you sit down to play there is nothing else in your mind but playing poker....find and acknowledge your limits time and attention wise and stick to it....very simple really.