PS3Center recently got the chance to ask Deus Ex: Human Revolution's Game Director, Jean-Francois Dugas, a few questions. The full interview will be posted soon, but one of the interesting pieces of information he let us in on was that the game should last a minimum of 20 to 25 hours.
"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."
So it looks like the game is definitely living up to its RPG roots.