Hello Norbee,
Could you please give little more detail on your question, so we can answer exactly. As Pavan mentioned you can run reports for Client session, and if you want run for a specific group or folders, you could do that too, where reports will show the clients that connected to the devices in the respective groups or folders, you can select multiple groups and folders as well, similarly you can also run the reports, based on the ssid, networkchip vendor, Vlan, Roles.... etc.
You could do this by clicking on report definitions, click on add and select the report type as client session and use the filters accordingly.
However, as you mentioned "Client's session time" in the subject,I am not sure if you meant to run a report based on the session time, that is not available today, but you can pick just report without any filters or session data by ssid or device.. etc, and run the report for the particular time you are looking for.
hope this helps.
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