Average wait duration is a function of the CSQ that you're doing the Get Reporting Statistic on, and the CSQ doesn't see the call till you do a Select Resource. So it is only calculated using actual time spent after the Select Resource step.
Also, note that average wait duration will just return the average time someone spent in queue. This may be what you want, but if you want to find out how long we expect them to be in queue, use the Expected Wait Time Field. It is able to give better estimates by taking more variables into account such as Contacts Holding, Agents Logged In, etc...
First time I used this, i had people in queue hearing that it would be 1 minute (the average wait time) even though it was a bank on a Friday payday and there were 15 waiting in queue during lunchtime and the expected hold time was closer to 10 minutes :)
The reason I am looking for this average value is for reporting purposes.
This customer is graded on a time to answer metric. However, anything that happens before the select resource step shouldn't be included in this metric. Their reports are showing the entire length of the script, not just the queue length. I was hoping that the Avg Wait Duration would give them the information they need without having to create some crazy script in multiple parts with some fancy way to transfer caller entered data between the call redirects. Less complexity = better.