The length of a game is always important, especially when multiplayer isn't a viable means of extending the experience.
And although Bethesda typically produces some of the biggest and longest games in existence, one has to remember two things concerning their new title, Dishonored : Firstly, Arkane Studios made it, and secondly, it's not a giant open-world sandbox adventure like The Elder Scrolls .
Therefore, we shouldn't be surprised to hear early length estimates coming in at only 12-20 hours. That may seem a little low to some ardent gamers and in fact…it is. According to a post made by level designer Dan Todd in Bethesda's official forums , the game will take a minimum of 22 hours.
"My ghost and no kills play through took 22 hours. That’s without exploring every nook and cranny, and knowing the solution to everything beforehand. It should take more thorough/inexperienced players a bit longer than that."
So if you don't happen to know the solution to every last situation and you weren't involved in the actual creation of the game, maybe 25-30 hours is more accurate. It's also true that you can play through this adventure any way you choose; hence, two play-throughs don't have to be exactly the same… So experiment to your heart's content!
Related Game(s): Dishonored