Tracking First Response on Case Record

Tracking First Response on Case based on Case Comment 

This is common requirement to track the First Response time on Case based on the first Case comment.

Here is the simple Solution:

1. Create a custom field of type Date/time called first response time on Case object.

2. Create a workflow rule on "Case comment" object"  with criteria where first response time is NULL.

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3. Create a Workflow Field update on this rule to update the first response to NOW().

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4. Finally Activate the Workflow Rule.

Hope this will help you.