I got my card immediately after filling out their information sheet.
I'd assume that since the card is basically a gift card that is worth as much as a membership (they sell you gift cards around the holidays if they run out of membership card packets) and since the gift cards have no expiration, then I'd assume the membership gift cards also do not expire. They only hangup I can think of is that if the cost of membership goes up, they'd have to pay the difference. I cannot find confirmation of that online, so you'll probably have to contact Costco to verify.
You should call them and ask. I really don't know. Sorry
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