My longest gaming session was probably on League of Legends. Back when I started playing in high school, when school work was still easy and responsibilities were non-existent, I think there were days in those first couple of months where I'd wake up, log in, and keep playing until my eyes gave out and I had to sleep. It's probably at least 12 hours of screen time, and I don't really remember stopping for food or anything really...
Thankfully I'm way past that stage now. I do find it a little sad that my backlog of games seems to grow faster than I can get to it, but the most games I'd play in a session is for a couple of hours, possibly made up of multiple of them in one day if I really wanted to play a certain game or something (couple of hours, go away and do something else, come back and game for a few more hours, repeat until life comes calling again). It's probably not the most exciting lifestyle for a self-proclaimed gamer, but I find myself enjoying those sessions so much more because I only play when I feel like it and have no chores or commitments nagging me on the back of my mind.