It's common thing for exploration and mystery games to have long play time. The time it requires for player to explore and unravel mystery is usually longer than action games. IOW those games are slowly paced.
Unfortunately, the same couldn't be said about horror. Horror couldn't last long cause player would adapt to the atmosphere after some time.
Oh about your main question : my games are usually action games therefore they last for 1 hour at max. If it requires longer than that, I'd implement save system.