It depends on the story and what you personally like as a gamer. I wasted 60 hours on AC: Brotherhood because I LIKE dicking around Rome and completing side-quests, etc. Most people just want to go through the single-player campaign and then stop after it ends, but I personally like playing on and exploring the sandbox.
Not everyone likes that so some people will think it's too long because some of the side quests are mandatory. It's all based around what the player themselves chooses to do between the missions/goal of the single player.
Not everyone likes that so some people will think it's too long because some of the side quests are mandatory. It's all based around what the player themselves chooses to do between the missions/goal of the single player.