Interesting point: How long a game takes to play through is significantly dependent on the person playing it.
Take my brother. He always takes longer to play through games than I do, usually between 30 minutes to 5 hours longer, depending on the game. This has nothing to do with his skill level, and everything to do with his style of play.
I personally like to blaze through games. I see what I need to do, I get straight to it. My brother, on the other hand, is an explorer. He will examine every square inch of an area before moving on. He will sit back and relish in the sights, the sounds, the atmosphere.
Does his increased play-time mean that he enjoyed the game more than I did? Well, yes, in most cases he does derive more enjoyment out of a game than I do, because he pushes himself to find more to enjoy in each game.
My point is, most games don't have a fixed, definitive amount of time in which you can derive enjoyment from them. Granted, not many gamers would really enjoy the level of exploration that my brother does (I know I would get bored of it rather quickly), but at the same time most people could find something to do with a game that would allow them to enjoy it for longer.
For example, many people will get more play-time out of an RPG by running through it again with some self-imposed constraint. Perhaps they'll only use the starting characters, or they'll refuse to purchase better equipment. I have a friend who, whenever playing an RPG, will always keep his characters under-leveled, thusly forcing him to act more strategically with items and skills, and inevitably increasing the amount of time he spends on each battle.
Then there are the people who do speed-runs, trying to finish the game in the least amount of time. These people will naturally be re-playing their chosen game many times, perfecting their movements for greatest efficiency and constantly seeking new ways to shave off precious seconds.
So really, the 'length' of a game as a factor in determining how much value it has is only as important as a given player allows it to be. If a player can discover a new way to play the game, another way to increase how much time they can spend enjoying it, then its value to that player increases.
So the next time you find yourself bothered that a game wasn't long enough, or that you didn't get enough quality-time for you money, just try giving it a bit of thought to see if you can find some way to get more out of it. And if you can't figure anything out, perhaps you should be consider being a bit more selective in how you spend your money.