Recipe for average time from Unassigned to Assigned to an agent, specific to a ticket group?

Hi All,

I'm trying to find an optimal way to determine how long a ticket stays unassigned within a certain ticket group, on average.  The workflow is as follows:

1.  Ticket submitted

2.  Ticket picked up by Tier 1 Support

3.  Ticket escalated to Tier 2 Support (group level, unassigned)

4.  Ticket is assigned to, or picked up by, an agent within the Tier 2 Support ticket group

I'd like to determine the average time it takes to get from #3 to #4 for all tickets daily.