your looking for a unix command to delete files older than a certain date. there is nothing out-of-the-box that does this so your files stay there indefinitely.

for recorded calls I run this out of crontab

@daily /usr/bin/find /var/spool/asterisk/monitor/ -type f -mtime +60 -exec rm {} \;

that deletes it after 61 days. If they cant copy that recording off by 60 days, I have no sympathy :-)