If you're talking about the code download I just did it myself. Only took an hour and a few more minutes for me to download the full 6.5 gigs. Someone might be using up bandwith where you live, not sure.
If anything check to see if anything is bogging down your internet.
Yup, Daldieb got it right. It's about a 7 GB download (which is huge), and if your internet connection is sub-par or extremely active at the time you're trying to download, you'll have a lot of trouble downloading it. There is currently no service alerts
for Xbox Live which means that everything (including the Marketplace) is working properly. This means you can narrow it down to a problem on your end. If you feel that 15% is worth losing, you can attempt to cancel the download and restart it.
Shut your Xbox off, turn off your modem (and router if using wireless) for about a minute. Then turn on the modem and wait for it to fully boot, turn on the router (wait until fully booted), and then turn the Xbox on and connect to Live. Then try redownloading
it. Make sure that nobody is downloading large files (especially streaming from websites such as YouTube) while you're trying to download the game, especially if your network connection or connection speed isn't great.
That might help. Alot of people kept telling me to get off wireless and use the ethernet cord. For some odd reason it actually helped my downloads and online matches. I didn't have DC issues anymore nor did I lag horribly in multiplayer.
Though in the end it could honestly be a number of things. Good luck finding the problem. Only advice I can give is go to google and start researching. I know it might take awhile but at least it would help figure the problem out :D