Depends on the aging policy applied to the user.   Verify which policy is applied under the end user  Edit \ Mailbox.  Then look at that aging policy under the Message Store \ Message Aging \ Aging Policies.   There should be a section for Permanently Delete Messages in the Deleted Items Folder.  This will tell you how long it saves tem after deleting them.   More than likely it is gone already, as I think the default is 15 days I think.  If you have a backup from when the message was left, and you have the resources, you can try a offline restore to recover.