Well. The last 5 games I bought (around £30 each) I never finished as I lost interest. Doubt I spent more than 2 hours on each. If Portal 2 is anything like the first one, it'll be 30 bucks well spent.
This seems to be a recurring theme.
Perhaps there is little appetite for longer games amongst mainstream gamers today.
Again I'm probably showing my age, but years ago 6 hours would have been considered a very short campaign indeed. Less so amongst FPS games, true. But if you look across all genres you'll find that PC games a decade or so ago took a lot longer to complete than todays "quality over quantity" releases.
Tefal asked which games lasted 15-20 hours. Hard to say because I haven't played them in ages. Perhaps I'll just list a few and we can come to a consensus about how long they took to complete?
FPS/shooters
Deus Ex; SS2; RTCW; Max Payne; Quake 2; Hexen; Heretic; GTA3/VC/SA; Far Cry
Other genres
XCom (any); most strategy war games ever made; most RPG games ever made (inc BG, FF); any sports game; many pure puzzle games & some platformers; Elite; Flight Sims
Then you have replayability. Some games offer almost endless replayability, such as XCom, Civ, San Andreas.
Heck, there SEGA Megadrive games that I got a lot more than 6 hours out of (Road Rash, Ecco, etc).