CoD: MW3 Campaign Fails to Impress; Fun for 3-4 Hours
Call of Duty: Modern Warfare 3 does not offer a long campaign, even by the series' standards.

Modern Warfare 3 reportedly requires only 3-4 hours.
Although the new part of the series will officially come to the market only on November 10, the game's campaign was made available already yesterday to those who pre-ordered it. To this we owe information on how short the single-player game is.
Four hours was enough for youtuber MarleyThirteen reached the credits. A similar time was achieved by JGOD, who was not in a hurry to get through the new CoD (via X). Both he and another youtuber - TheGamingRevolution - found the story not very exciting and too "safe".
The short time it takes to complete the game probably won't surprise anyone. Already Modern Warfare 2 from 2009 was widely criticized for its barely noticeable single-player campaign, and subsequent installments haven't changed much. To complete a typical CoD we usually need no more than 10 hours, and even that's assuming you're not in too much of a hurry (via HowLongToBeat).
Nevertheless, MW3 seems to be an overkill even by the series' standards. Last year's campaign from Modern Warfare II required spending about 8 hours in the game. That's twice as much as the latest installment, which, according to the aforementioned posts by youtubers - is by no means better in of intensity.
Perhaps the reason is that MW3 reportedly began its life as a DLC for the second Modern Warfare. At least that's what was reported by Tom Henderson at the beginning of the year.