The longer the better.
If it's a bit shorter but has co-op I'm also very pleased (RE5, GeoW, Halo 3 etc.)

For me its about quality. If the game is a quality game length/replay don't matter to me.

I'd rather play a great game once than an average game with lots of unlockables, MP, etc. more than once.

If the game is great I don't care if its short (ICO, Portal, etc) or long (Deus Ex).

While of course more great content seems better than less, the enemy in long games IMHO is padding.

It depends on the game too - some are designed really as a single playthrough experience, while others are designed specifically for replay.

But quality I say, always quality first.

As long as its one of those. Either long or short with "replayability". Both are good in their own ways.

Shorter and more replayable.

SHMUPS, fighting games, action games, and the like are generally way more fun than longer games because you get right to the fun part immediately. There's no random wandering from point A to point B and no hassling with inventory screens or the like as you are put right into the action right away. There's no "here comes the good" part stuff because every part is the good part.

Of course, games that are utterly fun from start to finish are extremely difficult to find and everyone is different making it even tougher. Longer games will generally have more variety so most people will find something they enjoy even if they don't spend 100% of their time doing that one thing.

Where do games that feature great online multiplayer fall into? Short with great replay? If so then I go with that choice.

Are we talking about the 10 minute CS match or the 8 hour Seiken Densetsu III game played via an emulator?

To me story is the X factor for making a fun game, thus longevity and complexity of said story is key. A short game with large replay factor pretty much guarantees lack of an intriguing story.

Platforming was fun when I was a kid but even those games took longer than 4 hours to finish. I almost never replayed a game even back then to try and beat my previous high score. I merely did it because the game was fun for me and/or even back then some games had different ways to finish (multiple paths).

As opposed to playing for the intriguing story.

How uncivilized we were back then when we played games merely for fun...

Those types of games are no longer fun for me. I wondered why for a while but found they did not intellectually interest me enough.  You'll note I said a story was what make games fun for me now in the first line.

what ever gears of war 1 and 2 are...