Not sure if everyone has read the entire official rules on their site so I thought I would highlight a couple important dates.
PROMOTION PERIOD: The Mtn Dew Game Fuel/Doritos Call of Duty®: MW3 "Rank Up Your Game" promotion (the "Promotion") begins October 10, 2011 at 12:00:01 p.m. ET and ends December 31, 2011 at 11:59:59 a.m. ET
12. REDEEM YOUR XP: XP minutes must be transferred to a Gamer ID no later than 12/31/11 or they will be automatically pushed to the Gamer ID last used on the gaming platform chosen by the Participant on or about 1/9/12. If no Gamer ID has been entered during the Promotional Period, all remaining XP minutes will be forfeited.