FFXIII main story length is 50-60 hours long

"For XIII, the size of the entire game is considerable," he said, as reported by Kotaku.

"Just running through the main story takes experienced players over 50 hours".

He states that for experienced Final Fantasy fans it should take 50 hours to complete the main storyline and about 60 hours for beginners.

On top of that many side quests have been announced recently so it seems like this game will soak up many hours of your time.