This is unfortunately not possible directly at the moment. What you can chart is the KPI of user actions, but none of them measures the time the user had the page opened.
The only method I can think of is to measure the time passed between two or more consecutive user actions. But this is something that cannot be calculated right now directly in Dynatrace, but you can calculate that externally based on data collected by Dynatrace. For example select the required user actions using USQL and process the calculation externally.