In my jurisdiction: A gift certificate or credit memo sold or issued for consideration in this state may not have an expiration date, or expiration, period.
For what it's worth: this varies per country. In The Netherlands for instance a gift certificate has a minimum validity of 1 year if there's a start and end date on it. If there's only a start date, the validity is (at least) 5 years, and if there's no date at all, it's valid indefinitely (
source in Dutch). The catch is: It's allowed to refer to general conditions on a website for validity.
A judge can rule to end validity of this last category, as recently happened to old stamps (
another source in Dutch).