AUTH_ONLY transactions are only kept in your Unsettled Transactions for 30 days. If an AUTH_ONLY transaction is not captured for settlement within 30 days, the transaction status will change to Expired, and the funds will not be transferred.
http://support.authorize.net/authkb/index?page=content&id=A510&pmv=print&impressions=false Full list of transactions can be found here.
There is one more option to validate credit card AUTH_CAPTURE + VOID combination, that will charge and refund transaction.