Back in the day when I was attempting to get my grind on, I played 2-4 hour sessions with 8+ tables, take a break for food/Madden/NHL 09/etc for a half hour to an hour then back to it.

Now that I'm trying to not mass table and suck at poker I'm playing 2 tables and only going for 1.5 hours max at a time without taking at least a five minute break. I've gone over this more than once, but it was by less than 15 minutes so wutevs.

Just play until you feel your focus slipping, subtle tilt creeping in, you get bored, etc. One thing that kills my focus/concentration on poker and gets me bored with it is having the TV on or having people talk near me. It distracts me like crazy. However talking on Skype or having IRC open and off to the side so I can still see my tables doesn't seem to effect me so much.