When a premium publisher sends a code, they have the option to set that offer to expire after 7, 15, or 30 days. The creator has that amount of time to take action on the offer: accept or deny. If they deny the code, it is returned to your code supply immediately. If they accept the code, it is in their inventory but does not count against their quota (you offered it to them, after all.)

If they don't take any action, then when the offer expires the code is returned.