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.
