Okay, a few things to clarify.
First, seven hours is how much time
I took on a first playthrough, in, yes, one sitting. I do not normally play through entire games in a single sitting. Portal was a special case influenced by circumstances outside of Valve's control. (Namely, my homework.
) Demetri took ten hours to complete the single-player campaign, according to his post on Veto's forum thread. I'm sure there are some people who will take less time than I did, considering I'd estimate that about an hour of my run was spent stuck on 2-3 puzzles or poking my nose around for easter eggs.
Second, co-op is a campaign and story entirely unto itself. That's another 6-10 hours of content, and it's probably a bit more replayable than the single-player campaign because of the dynamic of playing with another human.
Third: Achievements. Guarantee that the vast majority of people won't get them all in a single sitting, and there will be plenty of achievement whores trying to get them all. I know I missed a couple from the single-player campaign, and intend to go back for them.