Performance based policies set to evaluate over a range of days need to have the conditions met for ALL days consecutively.
For example, a condition set for "Avg CPU under 20% for 5 days" must have an average under 20% for 5 consecutive days before the policy will trigger. If a single day within the timeframe does not meet the condition, the policy evaluation will reset.
It is also not retroactive. If the above policy condition was created yesterday to check for Avg CPU under 20%, it will start evaluating that day and not flag an instance that had been under 20% for 20 days prior to the policy’s creation, for instance.
For more information on policy configuration, see: Build a Policy That Sends Notifications