Much like Deus Ex: Human Revolution, if you choose to goof around a bit and enjoy whatever the game has to offer, Dishonored will take more than 20 hours to finish. Dishonored is a first-person Neo-Victorian/steampunk game developed by Arkane Studios.
Bethesda, the publisher of the game, has mentioned that depending on your play-style you can either finish the game in approx. 12-14 hours or take twice as long.
“Play-style matters a lot. Very direct players will probably get through the game in 12 or 14 hours. Explorers who eavesdrop a lot will take twice as long,” their blog read.
“There are six major active powers and four enhancements (passive powers), in addition to weapons and gadgets; plus there are 40 bone charms in the game that grant small supernatural perks. But no one can have all of these things in one playthrough.”
Powers like Blink and Dark vision will be free because a small amount of mana will regenerate allowing you to use them again after a while. Piero’s Spiritual Remedy (mana potions) are required to use the higher powers like Possession or Bend Time.