20 hours of game play is the minimum for a full priced game, though for me this rule generally applies to PC and console games which are in every aspect supposed to be bigger. I am not going to go out and slap down 80 dollars for a game with a stupidly short single player mode, especially shooters. I was pretty content with Borderlands though, at 50 bucks with all the DLC available at the time, I put in 24 hours just on my first play through without doing any of the additional content. I am on my second play through now and am doing the extra stuff as I go and trying to get 100% by getting all the missions completed. I estimate I will put in a good 60 hours to complete everything after my second play through is done.
The length of a game does matter, 20 hours is pretty short and I don't mind it so much for portable games, having 20 hours just to play through the story or whatever and then having lots of online and extra things to do (Pokemon is a prime example), it winds up being considerably longer then it should be. I think people also, especially the older gaming population, tend to equate the length of game time by it's difficulty. Back when we had no saves and games were actually pretty hard (Contra anyone?), the length of time you would have to spend to beat certain games equates to being about the same as easier games that are just naturally longer because you can save and don't have to start from the freaking beginning when you lose.