Certainly I want be able to play for only 30 mins before I can save, but I do like my games to last a while. In terms of online/offline, if it has a robust online mode, then I am happy with a short, but sweet offline (CoD). Sometimes a short game can be very good, if it is a game that you want to play again. The only problem with a game like Uncharted, which I beat in about 7 hours (+ deaths), is that it was so memorable, I can remember every level, so for the meantime I want to leave it for a bit, before it becomes less familiar. R & C was quite short, but it was so fun that I played it through again immediately. However, for many games what I value is replay value, as I do not like to waste my money, only a game about 20 hours can get away without replay value

Depends on the game, some i like short some i like long.

Some people have short attention spans. So for some people... around 8 hours and they're done with the expierence. They want a short game because they want to beat the game badibing and be done with it and not pick it up for months and not have to remember anything about the story.

Of course... the people who are like these don't tend to be the "casual" group.  More people who have grown up in the "low attention span" generation.

I prefer games that are Longer because they tend to have a good story and because I want my moneys worth. If I need a short game for some reason I will just rent it because I cant see spending 50 dollars and finishing a game under 15 hours

It all depends on the genre. For anything in the vein of action-adventure, beat 'em up, FPS, or platformer, 8-10 hours is about what I like. Anything more and, unless the game is something special, I start to get bored.

For Zelda-likes, I can do more along the lines of 20. For RPGs, maybe 30, or up to 40 if the game is really good.

For horror games, my threshold is lower - more like 6 or 7. You don't want horror games to be long, because they can quickly cease to be scary if they're too drawn out.

As for gaming sessions, I prefer 30 minutes, at the most. I like being able to drop a game and still save my progress in a short time if I have something going on with friends or classes. 

Remember that quantity does not equal quality, and that many longer games are simply padded with filler. I'd rather play a tight, balls-to-the-wall 8 hour game than a dull, scattered 20-hour game.

Call of Duty 4 single player campaign was way too short. I completed it in 3 hours. Longer games for shooters and RPG's are definitely better, more value for money, providing the game is enjoyable. Games like Oblivion 100 hours plus and Super Mario Galaxy and Mass Effect both around 40 hours. Bioshock was dissappointing being only an 8 hour game.
Having said that it really depends on the type of gamer. Casual gamers and Achievement whores prefer shorter, easier games to play. Hardcore gamers prefer longer challenging games to play.

I prefer long games (as long as they aren't too repetutive or games that you can keep coming back to.

Doesn't matter to me either way. As long as the game is good.

Short game with good replayability = Bang for my buck.
Long game that takes forever to beat = Bang for my buck. :P

Both have they're place, but I generally prefer long games.