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

Sample Report: