5.8 As of March 6, 2015, redemption of promotional credit is limited to 12 months from the date of issue (unless otherwise stated), at which time the credit will expire. Upon redemption, promotional credit expires after 12 months unless otherwise defined in the terms of the promotion. For instances where promotional credit was issued or redeemed prior to March 6, 2015, that credit will expire on March 6, 2016.
tl;dr if you have free credits such as the github student offer etc. use em now