Dan Todd, Arkane studios level designer, confirmed on the Bethesda official forums that the upcoming game Dishonored will have a play through time of longer than 22 hours. While replying to a forum user he posted:
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
Note that in Dishonored you can complete our objectives in various ways so that also adds to the replay -ability of the game and its longevity. More than 22 hours for an action game is a quite big figure in this day and age especially, don’t you think so?
Here is the link to the forum post.