Need a way to run a report that shows time a ticket was with a specific group.

My build is: 9.0 Build 9039

So they way our team works is that Tier 1 gets the initial request and then they may escalate to Tier 2, then it may go a step further and get escalated to tier 3.

I would like a report that shows how long tickets were in each stage.

I know it tracks it in the history of an individual ticket, but I can't seem to run a report that would show me this information in an aggregate way for all tickets. 

Please advise.