question about unused gophone refill card expire date

Hi, I bought two gophone refilll cards ($25 each) from target store. i used/ loaded one refill card, it will expire in three months. What about the other one I have not used/ loaded to my gophone? When will it expire? can I use/load it after three months? in the target store receipt, which mentions it can be returned within 30 days of the ship date.

Thanks a lot,

Sean