The Guardian made an interview with one of the people behind Batman: Arkham City, Sefton Hill. When asked how he planned to retain player interest, he spilled that the game would take 25 hours to complete the main storyline, an extremely impressive number for these kinds of games.
“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.”, what the story fully entails however is limited in its information. It takes place one year after Arkham Asylum, and Quincy Sharp, the previous warden of the asylum, took all the credit for stopping The Joker, using his newfound reputation to become Gotham City’s mayor. However, the conditions of Arkham and the Blackgate Prison make them unsuitable to house inmates. Quincy put his money to use and bought himself a private military group and several slums in Gotham City, dubbing the new area as Arkham City. The prisoners can do what they wish in the city (except for escaping), and Batman keeps a weather eye on it, seeing it as an incredibly bad idea.
The actual story however is caused by Two-Face. Deciding to make a name for himself, he plans a public execution of Catwoman. Batman, seeing this as a very bad thing, makes certain he will stop Catwoman’s death and bring the villains to justice. That’s the general idea and backstory about it from what I can gather.
He went on to say that they took great care to carefully introduce new mechanics at a pace where players wouldn’t feel overwhelmed. Players new to a game can be easily put off by an extremely large amount of ways to do things, rather than being given new abilities as they become relevant.
As for the other 15 hours, that will be eaten up by 15 hours of story off the main path, and can be tackled by the player whenever they wish it. This totals a length that stands up to most RPG games that I’ve played going through the main story alone, like Final Fantasy X or Xenosaga 1. Most action games wind up being clearable within a few hours, but it’s likely this will be a bold exception to the rule. Even though it’s not due out until another 3 months, it’s looking and sounding to be a solid candidate for game of the year.