Reporting: First Reply time/Second reply time of an end-user

Feature Request Summary: 

To be able to mesure First reply time of an end-user when we create a ticket. 

Description/Use Cases: 

In a project, I would like to measure how fast end-users are replying to us. 
Let's say, if we are doing a promotion project and would like to see the reply ratio and reply time of end-users.