[Dovecot] Purging Deleted Messages

I noticed today I checked a users mailbox with the mutt command:

mutt -f /home/bob/email/

There I saw he had 100 messages that had been deleted but not yet purged from his inbox so they're still using up disk storage on my email server. My question is does Dovecot have a process to automatically delete these messages after a certain period of time? I would like to think that they don't just sit in a users mailbox for years waiting to be purged, right? Does anyone know exactly how this works?

I checked my own mailbox using mutt and I see 11 messages that are deleted from my Thunderbird client but are still on the server (marked for deletion).

How long do these messages stay there?

-- Man your battle stations...