I hope it's not too late but I just realized this. About a year ago Ozbargain was inundated with deals from Amazon for free coins, e.g., Free Amazon Credit. The promotional coin expires 1 year according to the T&C:
"Promotional Coins expire one year after the later of the date we give them to you or the date you last purchased or redeemed Coins."
So if you haven't spent any coin and would like to keep it active, simply purchase anything to extend the expiry date. I have a live chat with Amazon and confirmed that as long as you made any purchase, all your promotional coins will expire a year later from the date of your purchase:
http://files.ozbargain.com.au/upload/86425/31192/amazoncoin.…
Hope this helps.
That explains why my coins haven't expired yet. Thanks for the info!