If you take the time eo enjoy every aspect of the game, I would guess 15+ hours. If you rush through the main story and ignore side quests, silver keys, gargoyles, and demon doors, you can probably beat the game in less than 7 hours. (I'm on my second playthrough of the game, my first time I beat the game in 10 hours, but ignored demon doors and silver keys for the most part- not to mention there was more things to do after I completed the story...which I didn't do.)