How often does the Trash get emptied?
Messages in the Trash have a 30 day expiration date. They can be removed from the Trash using the Move To function (your inbox is one of the Move To options if you do not want to label the message). After 30 days, messages are deleted and cannot be recovered.
To permanently delete individual messages before the 30 day period is over, select them and click on Delete forever. To delete all messages in the Trash click on Empty Trash now.