Interesting topic - how to quantify something as subjective as how long a game plays! There can be reasons why a game takes longer that don't necessarily make it more fun to play. I'm thinking of those games where you have to run here, run there, do this errand, pick up that bobble, hunt for pixels -- those kind of games may take a while to play, but can be very irritating. On the other hand, an intelligently designed game that makes sense can be played fairly rapidly because it isn't full of a lot of nonsensical padding.
When I played the first Myst game (lo those many years past), I had no on-line gamer contacts, no walkthrough, no Game Guide, and had to figure out not only how to play Myst but how to play an adventure game itself. I think I played Myst for almost four months. I loved it!
In general, though, I agree with those who say it isn't how long it takes to play a game that matters as much as the quality of the game. Having said that, I do remember feeling that Amerzone was "short." My playing style, since I still work three days a week, is to play for an hour or so in the evening and maybe a few hours on the weekend. A game may last me a month that way. If I got through it faster than that, I would be disappointed.
When the only tool you have is a hammer, every problem begins to resemble a nail.