That's pretty much it. How many singleplayer games can you get 40 hours out of? Or half that?
Depends on the length of the entire game and also how many times you actually decide to replay the game.
Many RPG's, you can bank some good 40 hours out of for ONE run through -- usually, that's if you also decide to do a lot of the side missions/quests, especially in more recent years. I can list many of them -- Fallout 1,2,3; Planescape: Torment; Oblivion; Morrowind; Witcher; Baldur's Gate series; Arcanum; Vamp: Bloodlines; etc. Often, many of these games, have different endings to the entire game, as well -- so having a 2nd walkthrough can be much different than your first. So can running a 3rd, 4th, or 5th walkthrough...
In terms of shooters, good luck with finding a shooter running you 40 hours on one run through. Usually, takes a few times to run through. Deus Ex can take around anywhere from 30-40 hours to finish, depending on what you do and how much of the side stuff you do. Run through Bioshock twice, to see both different endings, that should give you around 40 hours.
STALKER: SOC's main quest should take you anywhere from 10-15 hours to run through ONCE -- but chances are, you are going to spend a lot of time doing more than following the main quest b/c of the way the game is built w/ now and then popping/generating random missions on your way all the time and b/c you're going to need to get better equipment to survive to get further through the game and its huge open-world.
L4D can be played through in 4 hours for all 4 campaigns. Meaning, you'd run through it some 10 times, at least -- if you want your 40 hours worth. If you're a person who runs through games once and moves on to something else, you'd not be happy with L4D. The endings for each campaign -- usually, you basically get away and you get the stats of your run through; there's nothing more, nothing less. Really, not much to it per run-through, now is there? And your characters here, no matter which one you are, they're not much different than each other, in terms of gameplay and style -- and, there's no multiple different endings to each campaign. It's quite the linear experience.
I ran through Shadow of Destiny some 8 times at least, to see all the multiple endings. That took me about 30 hours to get all of these different endings, by the way.
L4D, in comparison to many other games at that full price, really doesn't pack much quantity for the its original $50 tag. Regardless, what's there is great -- just, is it really worth its pricetag in terms of share content? $30 seems more reasonable, since what's there seems more like -- in terms of content, it's the size of an expansion pack.
Call of Duty 4, sure -- took me around 10 hours to finish the SP -- yeah, I died a lot! Many gamers, it took 4-7 hours. If you just did the SP only, you'd be hella' disappointed with that kind of shortness. Especially since it's linear and there's only one path here. If you came for both SP and MP or you just plan to spend a lot of time w/ MP -- which got some DLC and extra content (on the MP side) -- you'll probably get your $30-50 (retail price of what this game normally goes for, these days).
EDIT:
FEAR 2 PC -- another game, took me some 15 hours to run through one time -- b/c I died a fair amount and b/c there were some sequences where autosaves where (unfortunately) spread out a bit for me. Sure, most drilled through it in 8 hours, give or take. The SP content is pretty good -- but, is it worth $50? Not for the SP alone. Worth $30 for SP alone? I'd say yes. I didn't mention the MP at all really b/c -- well, nobody is freakin' playin' this game on the MP side over Steam, even though it has a bunch of different game modes. BTW, Armored Front MP freakin' rocks -- I must say. I wish on the MP side, they didn't leave out Slo-Mo DM, which I loved in the original FEAR. If you bought FEAR 2, you likely bought it for the SP, not the MP.
