If you want some examples, that’s what I’m using in my setup:
# Delete mails from junk which are read and older than 4 weeks
/usr/local/bin/docker-compose exec -T dovecot-mailcow doveadm expunge -A mailbox 'Junk' SEEN savedbefore 4w
# Delete mails from junk which are NOT read and older than 8 weeks
/usr/local/bin/docker-compose exec -T dovecot-mailcow doveadm expunge -A mailbox 'Junk' not SEEN savedbefore 8w