Some conventional wisdom suggests that if a couple divorces within a year after their wedding day, they should return their wedding gifts. But most people would agree that a gift does not come with "conditions." Once a gift is given, the giver is never in a position to tell the recipient what they should do with it, no matter the circumstances.
So, while it might seem tacky to some for a couple whose marriage lasted two months to have a garage sale to unload their wedding day booty or to continue spending unused gift cards after the separation, there's no rule that says a marriage must outlast a trial period before the gifts are deemed valid.
Early-twentieth-century etiquette writer Emily Post agrees. Post once said, "A divorced couple does not give back any wedding gifts, although a woman whose marriage was annulled, and who never lived with her husband, should do so."
On the other hand, wedding etiquette states that a person has up to one year after the nuptials to give a gift. If you're invited to a wedding and you're convinced that the couple is doomed to quick failure, you can always wait a few weeks before giving a gift. If the marriage falls apart before you get around to buying a wedding present, buy yourself something nice instead.