From /etc/rc.d/init.d/rhsnd: <cite> # interval in minutes to connect to Red Hat Network. Any value below 20 # will result in blacklisting of the IP address. INTERVAL=30 </cite> This makes one think that it's technically possible to set $INTERVAL below 20, but source inspection reveals that it has a hard coded limit to 20 minutes in rhsnd.
an errata release of up2date will address the comment; additionally a config file /etc/sysconfig/rhn/rhnsd has been added where you can edit the INTERVAL value without touching the init script.