Do Gift Cards Expire?

I have always wondered this because each year for christmas, I always get some type of gift card from family memebers and friends, and I always take a few months before I actually use them, thats is just how it always goes with me Plano porcelain veneers. But I have wondered if I take too long, do they expire? I found out that the major ones like Target, Walmart and Best Buy do not expire and I think the ones that do will have an expiration date on the back or something.