There is an out of box metric definition that tracks the duration when the field 'incident state' changes:
Verify that the out of box metric definition exist and has not been customized in the instance
1) Navigate to metric_definition_list.do
2) Locate Number = MTRC0000004 and Name = Incident State Duration
3) Confirm that incident state is still tracked.
4) You may proceed to create a report of of this metric definition by using the table source: metric_instance