Hello Graybeard,
One approach is to report on Sessions and their duration. Using the DirectAudit PowerShell API you can search for Audited Sessions. You can then automate reports based on particular Users, Servers or duration of Sessions (ones still opened, ones over 8 hours, etc.)
We already did this for some customers as Professional Services, but you can have a look by yourself first. If you need assistance, contact us.
Hope that helps,
Fab