In the first five days since Halo 4's release on November 6, 1,116,882 people completed the game's campaign, or roughly one out of every four persons who played campaign at some point in that span.
Some 4.5 billion kills were recorded by all these players, roughly two-thirds of the projected population of Earth. About 13.5 million hours were spent in campaign alone, with another 18.9 million hours spent in the game's War Games and Spartan Ops modes combined.
The figures come courtesy of Halo Waypoint and 343 Industries, the game's developer. More stats can be found at the link.
The First Five Days of Halo 4 [Halo Waypoint]