re: How long should a game be?

I think it's really subjective. I know Destiny is touchy around here but I'll use that as an example. You can cruise through the main story in probably a few hours at the very most, especially with some help. Which I think is ok bc that's just the beginning of the game. Loot, quests, multi-player are all part of it.
For a game that's strictly all in on a single player like Last of Us, I think 8-10 hours of game play should be the benchmark