Do redeemed codes expire?

This article may answer your question http://docs.info.apple.com/article.html?artnum=302264

MJ

Princess,
I hope you used all your song codes before they expired. We had 36 song codes showing in our iTunes store credits -- even after the supposed "redeem by" expiration date -- so we thought that the codes had been "redeemed" by being activated and that they would stay in our account until we used them. Hah. It turns out that the # of song credits that appears in the iTunes bar only updates when you log in to check your Account Info. So when I tried to see when the expiration date was (and logged into my account info), the song codes simply disappeared. No warning. You would think Apple would at least let you know that your credits are going to expire. I had tried to clarify this beforehand and spoke to several different people at two different Apple stores, and nobody knew the correct answer.
Bah, humbug.

The do let you know by telling you ( or the original guy in his case) the expiry date.

Does the dairy call you up to tell you you have a day left to drink that carton of milk in your fridge or do they print a date on it instead?

That link tells you:

Updating your credit count
To update your credit count, sign in to your account in the iTunes Store . Your updated credit amount appears next to your account name. Be sure to sign in to the iTunes Store on a regular basis to update your credit count, since your credits may have expired.