I'm a newbie on appdynamics but I need to implement a health rule for Red Hat for checking CPU Busy time (90 %) for a period of time (5 minutes) is it possible?
At first glance, it appears that you are referring to RHEL server monitoring.
Please see Server Monitoring documentation for details on tracking CPU Busy time.
Let us know how it goes. Feel free to post if you have further queries or concerns.