Discussed many times - one of the latest can be found here https://moodle.org/mod/forum/discuss.php?d=323036#p1318235
Core question - how do you measure 'time spent' on a webpage?
I frequently have my gmail, Moodle Themes forum and bbc (sport, especially during the cricket season) open all day. It certainly doesn't mean I spend all day looking at either of them. And when it comes to assignments - a student could spend weeks on a completing an assignment (e.g. a 6000 word essay) and about 2 minutes uploading it actually connected to the Assignment page of Moodle uploading that assignment. There are some plugins that claim to do this, but personally (emphasis that this is a personal opinion only!) I would treat the stats from anything like that with a very large pinch of salt (it's very easy to have multiple windows open, click in one periodically to keep it 'active' while actually having your attention on something completely different).