Uh....something like 12, I think. When Wrath of the Lich King came out, I did a "marathon" thing with a group of guildies to level. We made a 5-man group and just did quests/dungeons together.
That happened back when I was in college. I can't do super long gaming sessions like that anymore. My typical sessions these days are maybe a couple of hours at a time. Even in a game like Skyrim or WoW, I rarely play longer than that. Maybe a bit longer on off nights when I don't have to work the next day and can stay up a bit later.