How to know the idle time of an user in the web

There really isn't a way to do this out-of-the-box... 

Adobe's time metrics are very rudimentary.. simple math applied against page view timestamps.

• Page A (timestamp 10:01am)
• Page B (timestamp 10:03am)
  • Time Spent on Page A: Page B timestamp - Page A timestamp
  • 10:03am-10:01am = 2 mins
• Page C (timestamp 10:07am)
  • Time Spent on Page B: Page C timestamp - Page B timestamp
  • 10:07am-10:03am = 4 mins

If Page C is the last page in the visit, there is no timestamp to calculate the time spent...

You can see that there is no distinction between "Active on the Page" or "Idle", Adobe is not a heartbeat system, it is not checking constantly for activity (is the user still there? Is the Tab active or not? Is the window open or minimized? etc)

If you attempt to create custom code to detect this (using s.tl actions), you will inflate your server calls and could go over your contract allocations.

If you really need in-depth time spent, I suggest that you look into something which uses heartbeats and takes active vs idle into account... 

Or if it is one specific page, you could maybe use something like the getTimeToComplete (https://experienceleague.adobe.com/docs/analytics/implementation/vars/plugins/gettimetocomplete.html?lang=en) or getTimeBetweenEvents (https://experienceleague.adobe.com/docs/analytics/implementation/vars/plugins/gettimebetweenevents.html?lang=en) plugins to create some sort of custom timer tracking (assuming that you can identify the moment the user went "idle" and when they come back.