Should be based on when the Tech creates a Tech Note, not when the ticket is assigned.
Since I assign all tickets to our techs, all of our techs have an 'awesome' 1st Response Time, making the report virtually meaningless.
If the 1st Response Time was based on the 1st Tech Note, and the Work Time based on the 1st Response Time (or 1st Tech Note) to Resolved or Closed, the reports would be more meaningful.
Ideally, if the Ticket were put into a Scheduled State (or anything other than Assigned), the Work Time should be 'paused' as well. This would make a report on how long our techs work on tickets be meaningful.