It depends on when you picked them. There should be a clear expiration date when you view them on line. Go to something like profile, manage my ecerts after logging into your DL FF account. There's a column for the expiration date on the page that lists all of your certs.

Note that the expiration date is when the cert must be applied--i.e., upgrade space found and (normally) the ticket reissued--not the final travel date.