SD cards don't expire, but if you save your photos to your computer or cd you won't need that many... I usually have 2 on hand - one for the camera and one for a back up. The other thing you need to consider is if you get a different camera, they may not use SD media.
I know you didn't ask, but we will help you use up your gift card!
How about an oil change/tire purchase/rotation or DVD's?
Another consideration, I had a card once, for a store I didn't use, I sold it on ebay for close to face value.