To remove recordings older than X days (i.e. 3 days), SSH into your dialer and run the following campaign:
find /var/spool/asterisk/monitorDONE/MP3 -type f -mtime +3 -exec rm {} \;
find /var/spool/asterisk/monitorDONE/ORIG -type f -mtime +3 -exec rm {} \;