We believe you are looking for something like setting up a poll frequency only after alerts are to be triggered.
You can create escalation alerts to specific user groups. Please go ahead and create specific user groups and map one group for being alerted and the other for escalations alone. However, please note that this will apply for all the monitors.
(Admin > Configuration Profiles > Add Notification Profile > Contact Group for Status Escalation > Select group and time interval)
You can also set up persistent notifications wherein you have an option to be alerted after three or four continuous failures which is again an alternative that you can configure and see for yourself.
(Admin > Configuration Profiles > Add Notification Profile > Persistent Notification)