Easiest is to configure a new Metric Definition. You can find it in the ServiceNow Navigator under Metrics -> Definition. There already is a Metric Definition for the Incident state field. You could create a new one but for field State. After that you can report on it as @Saurabh Gupta suggests.
Note that this will not create records for past incidents but only for future incidents. If you also need historic data, your accessible option is generating it using a Fix script that leverages the sn_hw.HistoryWalker() API - it enables accessing all records (with auditing enabled) with each of that record's state. You would need to create a script that loops through each incident then loops through each update of individual incidents and - of course - compute the length of state and save it as a Metric Instance record.
Below is something I just created (by copying the existing Metric for field Incident state) - you can see that it already captured values as I played with an incident in my PDI:
