Honestly unless you damn well know how to make a game, you should focus on exploiting a few solid elements that work for a short time as opposed to making some long, overblown, drawn-out borefest.
So aim for an hour at most. Preferably 30-45 minutes. Hell, even less if that's all that's necessary for your game.
Don't be fooled by an industry that says you need 6 hours minimum for it to be even just labeled as a "short game."
Metal Slug can be beaten in an hour or less. And <img src="smileys/smiley35.gif" border="0">, you still play it even when you beat it.
When I'm playing a game on the computer that I hear about from some website, unless it has some SERIOUS clout (like Cave Story >.>), if I am not still entertained after 5 minutes I move on. I don't care if "it gets better an hour in" because it's still bad design, ha ha.
So I will say shorter is better for an indie game. Don't overstay your welcome ;)