Several commenters point out that a shorter game length better accomodates a busy lifestyle, with the completion of a short adventure proving more satisfying than the abandonment of a longer one halfway through. Predictably, several pundits argue that a $60 game should give you plenty of hours of gameplay and that it's simply a matter of getting what you've paid for. Such an argument has merit in extreme circumstances, but it also points towards some mysterious equation that could be used to calculate your game's value. Just how much should you pay per hour?
With largely subjective things like gameplay and enjoyment proving difficult to measure in meaningful ways, it's clear that a game's value -- which stretches beyond cost and length --is very difficult to adhere to a handful of dollars. Is an hour of Psychonauts worth the same as an hour of Devil May Cry? How many anti-meh's (see: The Algebra of Wows) per hour for each game? The best answer seems to be: length doesn't matter at all, it simply comes down to how you feel about the game. You'll know when you've gotten your money's worth, perhaps after spending just 15 minutes with the game.
Fun fact: This blogger gladly paid full price for Space Channel 5, a game that can easily be completed in under 45 minutes.