About 8 hours. That's okay with me. I think a 8-10 hour campaign is perfectly acceptable in modern games, because the multiplayer is the real draw. There's a statistic out there that Activision even recognizes, something like only 30% of CoD players even finish the campaign and another 30% never even start the campaign in the first place.
The campaign isn't the point in a CoD game. It's a bonus. The game is all about multiplayer, the campaign is the 'add-on' mode. It's a 180 from how it used to be, but maybe it started with Goldeneye 007. I mean how long did it take you to beat that? Each level was 5-10 minutes once you got good.
In comparison I've spent well over 100 hours in MW3 multiplayer.