The Ad-ID system refers to credits, which are essentially US dollars: $1 equals 1 credit. Codes start at $30, or 30 credits, per code.
If credits remain on the account at the anniversary date, you do not lose those credits. They will remain in the account and you can continue to use them during the next 12-month period.
Credits will only expire if an account is inactive. An account is considered inactive if there is no activity for a 24-month period, including an absence of deposits or withdrawals to the account. Inactive accounts will be closed, and any remaining credits will expire and be removed at that time. At no time are expired credits refunded to the account owner.