Sledgehammer Games stated on their Call of Duty Elite benefits page "All the MW3 DLC (20 pieces) during the 9-month DLC season" a
Option 1: They release the first map pack in January and the 9 month period goes through October; the PS blog hints at this because it says in its description of COD Elite that the first downloadable content will bereleased in Jan. 2012
Problem: This is a little close to the avereage release date of the next COD game and people will avoid spending money on DLC and save up for the next title.
Option 2: The first map is released in December and the 9 month period goes to September( COD XP for the new game was the first week of September for MW3 and I assume the trend will continue)
Problem: There are no sources that hint at this release and the majority of gamers willnot be active until after Christmas
Personally I believe Option 2 is more viable because the companies involved with MW3 will want to suck all of the money they can out of the game, so they would not want to release DLC to close to the next Call of Duty. Moreover, I personally would enjoy DLC as early as I can get it and I am sure others would too.