Probably somewhere in the range of 6 hours. Typically I play for two or three. There seems to be a cyclical rhythm, similar to a sleep cycle, when it comes to my immersion. At one hour in, I'm completely immersed. After about two to three hours, depending on what's going on, (in the game and in reality), my mind starts pulling away. I won't typically play at all though unless I have an hour, minimum. I've tried to play when I had just a 30 minute window, and it seems like I barely take a few steps before time is up.