How to calculate Revit Opened File Idle time in ..Net ?
The Revit API does in fact provide an Idling event that might enable you to get what you need:
https://www.revitapidocs.com/2020/e233027b-ba8c-0bd1-37b7-93a066efa5a3.htm
It has two modes. You can ask for the event handler to be called once only, when Revit enters the idling state, or continuously. Since no call-back is provided for exiting the idling state, you may need to implement continuous polling. That may have the disadvantage of degrading system performance. Alternatively, you may be able to determine when Revit exist the idle state from the journal file. You may even be able to determine both entry and exit into the idling state from the journal file.
Here are many other discussions of the Idling event:
https://thebuildingcoder.typepad.com/blog/about-the-author.html#5.28
Note that for many uses, it has now been superseded by external events. It may just fit your needs, though.
Looking forward to hearing how you end up solving this task.
You can file a wish list item in the Revit Idea Station if you think a different behaviour would make more sense.
Wow, that codeproject article looks really great! Thank you! Were you able to use it unmodified as is?
