How Long is Long Enough?

This is not a post about penis size, let's get that out the way now before all the comedians out there pipe up and get the two pence (cents) worth in.
I find myself getting frustrated with the length of video games, and it's a problem that has seemingly been present since I got my PS2 back in 2001, and has steadily become more of an issue since then. I don't know if it's just me that finds this, but most modern games have a very short length, which is something that I found to be an issue in both of my recent reviews for Wet and Halo 3: ODST. I find this to be an issue with a good 80% of the games I have played through in the last 7 or 8 years, even COD4 and Halo 3 were not as long as would have liked, it seems to be a common trend now that singleplayer campaigns average out at around 7 - 15 hours, which leads me to ask: is this long enough?
The games I played through prior to 2001 had much longer length, and while I wasn't anywhere near as involved in video games than as I am now, I played a variety of games across SNES, Megadrive (Genesis), Master System, PC, PlayStation and N64, all of which had some good sized games that kept you playing for hours upon hours. In my childhood, I spent hours on the SNES, playing games like Super Mario World, The Legend of Zelda, and even games where saving wasn't common place, they were huge adventures that would absorb hours upon hours of button bashing, brightly coloured, imaginative mayhem. But then again, time is relative to the observer, and when you are a kid time seemed to move much slower, so is this all a figment of my imagination? I also have to appreciate that my gaming sessions had time limits placed on them by my parents to an extent, so probably only lasted 1 or 2 hours at a time as opposed to the 5 or even 10 hour sessions I sometimes clock in at on a lazy Sunday these days.
I think not, because even today there are a few games that just seem to last forever, and a stark contrast exists between these long epic adventures compared to the short chapter like games that account for most of what is out there. Final Fantasy games can easily suck up over 100 hours if you try to accomplish everything on offer, and a couple of these are so good that you want to play them through a second, or even third time. The gargantuan offerings from Bethesda that take the forms of The Elder Scrolls: Oblivion and Fallout 3 both have had me clocking in over 100 hours each, and with such gigantic single player adventures such as these I can't help but feel other game developers are simply lazy and looking to make a quick buck.
It's a difficult to take a side on this argument, because I am also aware that there are limitations on technology, story, and ideas, as well as limitations posed by the type of games. when a shorter game is released, is it because the developer has spent their time trying to make a more solid gaming experience, so that what is offered may not be very long, but is of extremely high quality? This is certainly the case some of the time, COD4 and Halo 3 being the two examples used above that would also fall into this category, but there are also some very low quality items produced that are also very short in length, but maybe due to their poor quality that is a blessing, I've had a run in with a couple of less than amazing games recently (reviews pending), and I find that I can't be bothered to finish these.
The other factor that changes the shape of the argument completely is the presence of multiplayer, I may hammer my way through most FPS' in 2 or 3 sittings, but if the multiplayer is good, there really is no limit to the amount of time you could spend interacting and shooting other human beings, and I'm sure the same is true for many other genres such as racing games. But should a game rely on multiplayer? For me, multiplayer is an afterthought when I consider buying a game, unless I know ahead of time it's something that me and my friends are all going to get heavily involved in.
I don't really have a conclusion or direction to take this debate to, but if anyone has any thoughts or opposing views I would love to hear them.