This really depends.
Potty/commute/break: ten to fifteen minutes, max. Usually to provide a fun distraction, kickstart the brain in the morning or help slow down again after work. Basically the gaming equivalent of snacks.
Longer, and I'd be too engrossed, shorter would be too much hassle for basically no benefit.
After work/nighttime gaming: one to three hours. Just trying to achieve pig in filth status. I'm likely to fall asleep near the end. The point is to unwind, so no competitive stuff, barely any online stuff, no stressful games. Comfort food gaming.
Shorter, and I'd rather lurk gaf on my iPad, watch porn ponder the finer points of life, soak in the tub or read webcomics. Longer, and I'd rather be out and about.
Weekend gaming: usually local multiplayer, time required varies wildly depending on games, game type (competitive/CoOp), amount of friends (0,1,2-3,4+) and available/competing entertainment - but usually four to maybe six hours. Target: burn away stress and fatigue. Scream, laugh, rant and rave until nothing is left, or concentrate like fuck and soak in some mechanic or atmosphere. While stuffing face with delicious or unhealthy things. 404 No food analogy available.
Shorter, and it's not worth the hassle. Longer, and my loner tendencies and age start to offset the enjoyment from the noise. Not like I mind going over that limit (and I occasionally do when I'm trapped in the jaws of a new game), but that's generally when fatigue sets in.
In general though, if I had to choose one time span and one time span only, I'd probably choose two hours. Long enough to get shit done, short enough to fit in odd free time slots.
