Rocksteady has said that it'll feature 40 hours worth of main and non-main story content combined.
It told The Guardian that it'll feature 15 hours worth of side-missions.
"The Side Missions in Batman: Arkham City are a good example. We have around 15 hours of story that's off the main path," said game director Sefton Hill.
"It's completely down to the player when and how they want to tackle this – there's no right or wrong time. There's no other medium that can offer this kind of flexible experience where the viewer gets to tailor the experience they want themselves.
"In terms of narrative structure, I think games are much more analogous to an episodic TV series than a film. You have, say, 15 hourly episodes to fill, each requiring its own narrative arc but each plugging into an overarching storyline.
"It requires a lot of planning to make sure that every single hour is consistent with each other, is feeding and driving towards a compelling crescendo, while remaining implicitly self-explanatory so players can pick up and play at any time and know what they are doing."
Not content there, Hill confirmed a 25 hour completion timeframe for the game's main story.
"It takes over 25 hours to complete the main stories in Batman: Arkham City. To keep players engaged for this length of time, not only do the characters and the story need to progress, but the core mechanics of the game they are playing need to change and adapt as well."
Batman: Arkham City launches on October 18 in the US and October 21 for PS3, 360 and PC.