On the official Bethesda forum, Arkane Studios level designer Dan Todd has provided some insight into how long it will take to complete Dishonored once it releases next week.
Responding to a forum user, Todd said "my ghost and no kills playthrough 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."
Of course, as Todd mentions, playthrough times are personal and entirely dependent on how you play - especially for a game with as many different approaches as Dishonored - but these comments do suggest the game will not be short.