I get these codes by actually buying diapers for my son.. usually costing me $25 or more per box. I've seen 1 single Pampers Rewards code end much higher than my GIN with just regular bidding. You don't have to use GIN! Regular bidding is an option too. If auction ends with normal bids, then the second rewards code will be listed again with normal bidding. The 666,666 (Approximately $25) is how much it would cost me, per box to even provide the rewards codes to start with.
Expiration date is in auction details :) however, I have just added 5 rewards codes to my own account that were marked with a 2017 expiration date and all 5 still applied to my account.