For a story based game I'd say that 20 to 30 hours is reasonable. The original Deus Ex took like 60 (granted, I restarted halfway through...) and as much as I adore that game it was really too long.

That said, I do not play many story based games. I play an awful lot of strategy games, and I want hundreds of hours out of those! (And I usually get hundreds of hours out of them, too. Which is fantastic.)