How long do XBox Gold codes last?

Just wondering how long these last as I do have a 12 month Xbox Live Gold account but also got a couple of 7 days when I got Dead Rising 3 and Forza 5. But I go into redeem it but it says that you cant add it as I already have a 12 month one. So how long do these codes last?. Thanks.

Good question, Xbox Live Gold cards do infact expire. If you check on the actual card (I think it's the back) you'll see an expiration date.