Entering and Exiting are instantaneous events, so the rule will never break if a duration is added. If you would like to use Entering/Exiting, you will need to make sure that each zone is an appropriate shape to avoid false exceptions. This means that the zone must not overlap any adjacent streets.
One alternative is to use a combination of Inside Customer Zone and Device Stopped. This will generate an exception when the vehicle comes to a full stop within a customer zone. Your suggestion of Inside Customer Zone with a minimum duration would also work.
If an instant notification is not desired, the Customer Visits Report provides a list of each vehicle's stops within customer zones. This report includes arrival time, departure time, and stop duration. Reports can be scheduled to automatically email a specific person, or group of people, as frequently as every thirty minutes.
Please feel free to contact your Geotab reseller for further information, or assistance with creating custom rules.