How to set up Yandex Metrica for the time spent on the site?

Question

Yandex.Metrica

Good afternoon.
I want to know those who spent more than 20 seconds on the site. What should I use for this metric?

<script>setTimeout('yaCounterXXXXXX.reachGoal("TARGET");', 30000);</script>

?
But where to put it? In the footer?