The Avg. Session Duration is not so important for this. A session is the time people spend on your website, not just on one page. A session will automatically stop after 30 minutes of inactivity, at midnight or when a user arrives via one campaign, leaves, and then comes back via a different campaign.
The best thing is to look at the Average Time on Page. You can find it at Behaviour -> Overview. Or for individual pages at Behaviour -> All Pages. You can set the option Set row to 1500 and sort the column Avg. Time On Page from high to low. Then you can decide what Time on Page number you want to use. If many are above 15 minutes I would definitely set it to 30 minutes and see if many still reach the 30 minutes. If so, you could higher it again to 45 minutes. But I think 30 minutes is more than enough.
If you just want to let it go, you can use the option “I don’t want to set a Maximum Tracking Time.” I don’t recommend it, because in my experience you’ll get more accurate data but it’s not the data you are looking for. You’ll see that people stay two hours or longer on your page. Unless they are very slow readers, the chance is that they are not actively busy with your website. And are you looking for those numbers. You can of course filter them out in Analytics. I didn’t like the numbers when there was no maximum time, but if you’re willing to filter out high numbers in Analytics, you’ll have more accurate data.
I hope this helps.
