It's definitely 12 months, I was charged £32 and it shows as 12 months worth in my Shopto account. I don't have any charges showing yet on my bank account from Sony for the auto renewal. I just seem to have lost over a year of plus, it shows due for renewal on 10th February next year. It showed as valid for much longer when I initially redeemed the voucher, I checked at the time to make sure.
I definitely redeemed the code, I retried today after I got the notification from Sony, just in case, but I just got a message saying the code had been used.