This number completely depends on what your style of gameplay is; are you a straight-shooting main storyline man, or are you a lateral story, side missions and all kinda guy. If you're the former, the game should take you around 20-25 hours on the main story. However, this game is colossal, so if you're wanting to get the most out of your game, it'll likely take you far longer to explore everything there is to do and see.