How long is the free month offer valid for HP Instant Ink, and are there any conditions?

The free month offer is valid until 12/31/2015 and requires a promotional code to be entered at checkout within 7 days of purchasing an enrollment card or adding an eligible printer to an HP Connected account. The code is limited to one-time use and is applied to a single printer, and it cannot be redeemed for cash or combined with another offer. (Page 2)