To be honest, rather than focusing on length (I make long games), I think the most important thing is that the game be replayable—that is, different endings or variations on the ending, different builds, decisions that change the plot, etc. (you'll know what to do)
I think there's a big difference between telling a player... “The game lasts this long,”
and telling them... “You can play the game up to X times and you won't get bored.”
There's just no comparison—the second option is much better!