Arkane Studios previously stated Dishonored will take around 12-14 hours for players that kill every enemy. In a response to a question on the game's official forums, it looks as if the stealth approach is going to take quite a bit longer.
"My ghost & no kills playthrough took 22 hours," level designer Dan Todd wrote."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."
Which style will you go for?