Most of my offline groups are 4 hours. Enough time for a decent scenario or part 1 of a two part, but time left so it doesn't kill the whole day. When I play a very good strategy game, I'll play at a friend's house for 8 hours. Online here my first session I ran of One Ring RPG the players like it so much we played for 9 hours. But most of my games run for 4 hours. A narrative game like fiasco can be done in an hour or hour and a half. A tactical plus RP game like D&D 3.5 can be two one hour fights plus two hours of Roleplaying. Battletech takes typically an hour per 2 battlemechs involved, so that's a decent four hours if you play fast. A game of Twilight Imperium, or Battlestar Galactica can take a good 12 hours for the former, 6 hours for the latter if you roleplay out the rounds of challenges. I have wargames here that take over 100 hours to play.. you set it up and play once a week for 5 to 6 hours a day for 5 to 6 months. There are games that can be played in two hours, just not a lot here, but definitely possible. I just ran a James Bond RPG session that ran 3.5 hours. We stopped early because so much had happened, I wanted a chance to catch up with all the things the players did to the storyline. They made friends of enemies and enemies of friends, lots of new ways to spin things out. Good luck.