The thing about length is that we have an infinite amount of "fun" things to do, from playing great multiplayer titles, going through backlogs, reading, watching movies/tv, hanging out with friends, hiking, sports, etc. so my philosophy when it comes to length is make your work, whatever medium it is, as long as the length at which you can deliver nothing but your top tier A-game, because that's what I could choose to go do if your game ever dips below that quality; then you're making me spend my time in-optimally if you do make me play through less high tier work. For example, Dota 2 is always fun; in fact, it's almost always better than whatever else I could be playing to me. If you cannot bring your A-game past a certain point or the game starts to drag because you're going through redundant material mechanically (and, yes, I absolutely felt this way about RE4 at times) I shouldn't be playing your game then; I should be playing Dota 2.