[Dovecot] Purging Deleted Messages

[Carlos Williams]

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...