You have to factor in many things when weighing up the perfect length. Firstly genre; a fifty hour single-player action game would never get finished, an eight hour RPG would leave people feeling cheated. Secondly, variety; if you're doing the same thing over and over for twelve hours, you will definitely get tired of it by the final quarter. No amount of jazzy cut-scenes could prevent a repetitive stop-and-pop from leaving you with a dull feeling in your gut whenever you think about playing the game again. Thirdly, context; if you're trying to beat the game over a weekend, you'll stay on top of the difficulty curve but you might get exhausted with it by the end. Space it out over six months and you'll keep having to re-learn the controls and mechanics and wandering around not knowing where you should be (Final Fantasy XII, I'm looking at you). It's easy to just say that it depends on the game, but Drake's Fortune would have been more digestible minus the last hour or so of frantic blasting.