Tell me how to determine how much time a visitor spent on the site.

Good day to all! Tell me how to determine how much time a visitor spent on the site.

I have a movie site I want to make sure that visitors watch movies to the end. Advise the counter

Google analytics. There you set the time equal to the longest movie on the site (I don’t know for 3 or 4 hours) at which refusal will not be taken into account.

Automatically in the analytics the refusal is taken into account if the visitor is idle for 30 minutes

Every second, information went to the server that the user was still here. But this is subject to the presence of focus on the / body window. True, I'm not sure that this trick can be tracked, you need to experiment.