How to precisely measure the time visitors spent on my pages?
In order to accurately track the time spent on the website or single page, you can turn on a heartbeat timer in your Piwik PRO tracking tag. This feature is called Accurately measure the time spent on each page.
Alternatively, you can manually add following line to your tracking code:
setHeartBeatTimer( minimumVisitLength, heartBeatDelay )
You can learn more about the manual method here.