How to calculate the total time taken by a assignment group to close the HR case in a custom field

Hi team,

We have a requirement to calculate the time taken by the last assignment group to resolve the HR case,  from the moment the ticket was assigned to that group. I want to track the total duration from when it was assigned to that particular group until it is Closed we want to store the duration in a custom field. I tried with a metric, but it unfortunately captures the total duration for all reassigned groups as well. Therefore, we are considering using a business rule along with a custom field for this purpose. Could someone assist me with the script needed for this

Regards

Rak**bleep**h