Rocksteady's Dax Ginn revealed at Gamescom that Batman: Arkham City may take 65 hours to complete if the player goes through all of the side missions in addition to the main storyline. This figure is not inflated by the Challenge Maps.
"We’re talking a 25 hour main game here. Then, we’ve got at least 40 hours in side missions, while the challenge mode will take you who knows how long. And its in 3D!" said Ginn.
To compare, when Rocksteady was asked about Arkham Asylum's length before it was released, they stated it would take 12-15 hours to complete it 100%.
UPDATE: Rocksteady amended the statement to say that with doing the side missions, the campaign can be as long as 40 hours, not in addition to.
Dev: Batman: Arkham City 25 hours, with 40 hours more in side quests
24 August 2011 | By JonahFalcon