Hi John,
Here is the documentation for the report Time spent per visit :
http://microsite.omniture.com/t2/help/en_US/reference/index.html#Time_Spent_per_Visit_Report
Notice the following : "This report requires at least two image requests per visit in order to populate data. Visits with a single image request are not included in this report. Meaning, this report will not match the visits report for the same time period. "
So the visits missing in this report should be equal to the bounces I think. (A visit that consists of a single server call. For example, a single page visit is a bounce if a visitor does not interact with the page in a way that sends data to Adobe, such as clicking a link or a video start. If more than a single image request is seen in a visit, a Bounce does not exist. )
You will be able to see total of bounces in the key metrics report.
Best regards.
Alexis Cazes