I got a couple of GameStop cards from the USUM Year of Legends event and I seem to remember the codes lasting longer than a few weeks. Took me about a month before I finally redeemed my Yveltal, so you should be good for at least a bit. You can always just check the card for an expiration date -- or if you're really concerned, give someone you trust the code then have them trade you the Zacian/Zamazenta when you get your copy of the game.
