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Being the stereotypical "gamer" who almost never goes outside, I have no problem playing games for 12 hours each day. Sometimes all those hours are spent on a single game, sometimes I'll play two games... but I never play for less than two hours.
Like some other commenters, I just find it hard to get into a game if I'm forced to play in short sessions.

Of course it depends on genre and how gameplay is built, but for me and the games I play, less than an hour is usually not enough to “get into” a game. Then more than two hours simply feels unhealthy (though sometimes it is hard to stop, well-designed games can really get your attention). So I'd choose 1–2 hours, 3 at most.

It depends on the game, but rule of thumb for me is at least one hour. If I am not soaking in the game, I don't really enjoy it and I feel kind of outside of it, it's typically just feels like "uh, okay. I did some stuff... cool I guess?"

Some other games I really appreciate the pick up and drop nature of it. Like pokemon games, xenoblade chronicles 3ds, the alliance alive. But those are 3ds games more or less. when i'm on my PC. Unless it's a puzzle game where solving the problems can take a bit out of me. I like to sit down, and take it all in.

I feel like I'm always planning an "ideal session" that's 5+ hours, but realistically only play half that (or less). Life gets busy, it happens.

I'm sure I play longer sessions during the winter months.

Depends on the game, some I play short 15-30 minutes spurs each time and others I sit down and play for hours on end.

5h+ not anymore, usually 2-4h in one sitting, but if I have less than hour I rarely even launch the game

Sure, I can easily spend 15-30 with a casual game to relax or kill time. As for 'big" games, almost everything I play is story-driven, and it's a waste to play games like this in short sessions, so I won't start a game if I don't have enough time (1,5-2 hours or more).

When I was a kid & teenager, I definitely played more. Had more opportunity, energy, & some time too back then, though those dipped down during high school and nowadays. This summer / past few months I haven't played much, but when I do it's usually for 1 or 2 hours for a few days out of the week maybe. My Steam playtime hours is currently at '7.5 hours past 2 weeks'.

Unexpected from someone with a Zero Escape avatar, I can't imagine playing that in short sessions. First time I played 999 on DS, I played for 20h and did all endings in one sitting.