With Piano Analytics, you can have the time spent for a bounce visit thanks to the data processing and the events notion.
A bounce visit is a visit with 0 or 1 page loaded. (Or 0 to 1 "page.display" event recorded).
The time spent on Piano Analytics is calculated by the difference of time between two events.
To display the time spent for a bounce visit, this is mandatory to have a minimum of two events. For example, a visit with a page load (page.display) and a click on the same page (click.action).
In this case, the difference between the two events' timestamps allows the calculation of the time spent for this bounce visit.