There was a time, in university that my roommates, and I played 8-12 hour sessions most nights of the week and then we'd rp on and off all day 'between sessions' while we were doing other things, like drinking beer and cooking supper or doing chores. We would take a break from D&D to do annoying things like socialize with people, go to classes and drink at the bars. It was probably better to ask, "How many hours a day did you not play D&D."
Now that I'm married and have kids, games are around 3 hours or, if there's a big plot-relevant fight, it could go as long as 5.(but that's rare). I chose 4-6 because games are rarely less than 3 hours.