How long should a game last? Until it's over is the proper answer, I guess. But what's the right length for a game? The group of people that I play with meet primarily on weeknights, so we have some time constraints. While I love the idea of a game that takes six hours to play, it's just not very feasible. Maybe for a once in a while planned gaming experience, but certainly not on the reg. And there is something that bothers me about a five or six hour game. What happens if you are out of it after three hours, you just kind of hang around and go through the motions while the remaining players plug away at one another? Are you obligated to play the spoiler? Is it wrong the make moves that intentionally help another player, even if it's not the best for you? I don't know the answers.
I think for most board game a good time length is about 20 minutes per person involved, maybe a little longer if there are 3 or less. Five people and the game takes between an hour and a half and two hours? Perfect, I'm into it. An hour is a good length for two player games. This also opens the options of multiple games in a night, which I really like.
Our weekly D&D game goes around three and a half hours each week, which I find to be nice. As a DM it keeps me off the hook from having to prepare a ton of material for the game, but it's also enough time to cover a lot of ground and get a couple of combats in. In the past I have played games that meet for six hours or more, and that is also a lot of fun but I could never sustain that on a weekly basis. Maybe if the game met every other week or less frequently that would be the favored method.