In an ideal world, my gaming sessions would last 4-5 focused hours. I'm willing to settle for slightly less and could stay for slightly longer. More than than and fatigue sets in (mental more than physical).
Fortunately, in real-life we manage to stick to that right now, allthough less often than I'd like (once/month). We play at a house of some friends because they are the only ones with a child and they need to keep an eye on him. Actually, during the game session I have become the "babysitter" because he likes to come and sit on my lap while we play. It's distracting somewhat for me, but he's not too bad as long as we let him roll dice whenever it's "his turn" (which has no relation to the game).
However, I could stand a weekend of gaming, but I'd like the sessions to be close to my ideal. I used to play in the "Melanda Invitational" weekends. 4 sessions of gaming (Fri night, Sat morning, Sat afternoon, Sun afternoon) with an in-character feast on Sat night. With long breaks betweens sessions to recharge my mental batteries, that was ideal for me. A shame they morphed into LARPS and then completely away from the fantasy genre. That was usually my most enjoyable weekend of the year.