depends upon configuration - default invoca.ch rpm's would have (and thus RHEL/CentOS) distribution would have...
]# ls -l /var/lib/imap/db.backup1 total 9408 -rw------- 1 cyrus mail 144 Dec 6 09:20 annotations.db -rw------- 1 cyrus mail 9573343 Dec 6 09:20 log.0000000005 -rw------- 1 cyrus mail 21336 Dec 6 09:20 mailboxes.db
# ls -l /var/lib/imap/db.backup2 total 9336 -rw------- 1 cyrus mail 144 Dec 6 08:50 annotations.db -rw------- 1 cyrus mail 9500591 Dec 6 08:50 log.0000000005 -rw------- 1 cyrus mail 21336 Dec 6 08:50 mailboxes.db
thus there should be at least 2 backups of the mailboxes.db automatically. Of course this doesn't supplant a decent backup implementation.
I take it then that adding/removing mailboxes are the only times this file gets updated?