Sachys: Okay, asked Judas (whom it turns out is on holiday right now... O_o) about the codes and he said as far as he can remember they
should be valid for a limited time, but
best to get an exact answer from support (obviously he's on holiday, so can't expect him to goto the office etc and find out).
Nice of him to take the time to reply though.
-- Games that come in the form of a 16-character code are valid indefinitely - you can easily check that if you go to redeem one after clicking on "Continue", it's clearly stated so.
-- Games that come in the form of 16-character codes are time limited, which varies dependent on the promo event - for instance, the Realms of Arkania codes that where given during that speed test GOG did are valid till Oct 2014, but the codes from the Groupees bundles were valid for a much shorter time.