Dear Oodaddy2,
"Average Time On Site" represents the total amount of time visitors interact with a specific dimension item, per sequence with a dimension item. It is not just limited to “site” averages as the name suggests.
Since you have not selected Dimension(But Time Period), the numbers went to infinity as Adobe Cannot calculate the Average Time On Site with the list of dimensions available on the site.
If you select the Dimension (Includes month as well as Adobe captures the month also in Dimension), you will get the desired output!
Thank you!
Arun