The Arkham Knight story is of a considerable length. It can take quite a few hours to complete, depending on your playstyle and how much you explore the game's side quests and extras.
Yes, there is. After finishing the main story in Arkham Knight, you can freely roam around the game world. It's a great feature as it allows you to further explore Gotham City, complete side quests, find hidden collectibles, and interact with various elements of the game environment that you might have missed during the main story run.
The story length of Batman Arkham Knight can vary depending on your playstyle. Generally, it takes around 15 to 20 hours to complete the main storyline.