If you have a redeemable code it will expire the very moment you press the "validate" button.
At that point your client got the "ok" to give you your reward and the code is marked as "used by X".
If you decide for whatever reason NOT to choose your reward at that point and close your browser's window your code will be lost - forever.
FAQ:
Q - That was when my cat jumped on my keyboard selecting a grey nymph when I clearly wanted a purple one.
A - Enjoy your grey nymph
Q - I was jumping around filled with uncontainable joy and I tripped on the cord and unplugged the water cooling system of my extremely overclocked CPU. My CPU exploded.
A - Code is gone. Forever.
Q - I was about to click on my reward when my wireless, laser, atomic clock controlled, mouse ran out of battery.
A - Code gone.
Q - Could you please just check my account history and find evidence that, in fact, I did not eventually get my reward?
A - I could, but it would take me 20 minutes... That means I am not going to do it.
Q - But...
A - No.