In a recent interview with PS3center.net, director Jean-Francois Dugas revealed that the game would take players up to 20-25 hours or more to complete depending on their playing style. He said, "Depending on how you play it, if you take on additional quests, etc., it will make your experience even more expansive. Nevertheless, if you do the strict minimum, you’re still in there for at least 20 to 25 hours".
Deus Ex: Human Revolution will take players up to 20-25 hours or more to complete
Deus Ex: Human Revolution will take players up to 20-25 hours even if you do the bare minimum in missions