according to the website, you can redeem a max of 200 matches. you've bought and played 200 matches of double xp???
I just started trying it out. you redeem the code. then on the upper left of the web site you log into your xbox live account. then it'll ask how many of the matches do you want to apply toward your account. so a 12-pack of Mountain Dew gets you 6 matches. so i applied 6 matches toward H4. then after I finished a H4 round or match, it would double my xp. so instead of getting, saying 5000 xp in a round of Spartan Ops, I would get 10000 xp. Then it said I would be down to 5 matches left.
the suckiest part is if you have a terrible match. now I know why some chapters in spartan ops are popular and some have no gamers. I joined the first chapter of the fourth episode, I think, and there were so few enemies to kill, I only earned 2500 xp which got doubled to 5000. after that, I started playing the most popular chapters (which are much longer and have much more enemies to kill) to make sure I got 10000+ xp.