Microsoft has three map packs heading to Halo 4 in the coming months, and now it looks like we have exact release dates to work with thanks to an allegedly leaked dashboard ad (via The Halo Council, Destructoid). Assuming the info is correct, the Crimson Map pack is up first, on December 10, 2012, the Majestic Map Pack follows on February 25, 2013, and the Castle Map Pack arrives a few months later on April 1.
All of this content will be sold separately, though you can also drop 2,000 MS Points on the War Games Map Pass to get all three, presumably at a 400 MS Points discount (figuring on 800 MS Points apiece for a la carte purchases). The War Games Map Pass was also included as a bonus in the Halo 4 Limited Edition release, so make sure you don't have it before you spend any points on it.
Just keep in mind that these dates aren't concrete. Microsoft previously confirmed titles and release windows, but this banner ad has yet to be confirmed as legit.