The average page visit lasts a little less than a minute.
As users rush through Web pages, they have time to read only a quarter of the text on the pages they actually visit (let alone all those they don't). So, unless your writing is extraordinarily clear and focused, little of what you say on your website will get through to customers
Once the page has been loaded in the browser their is no more activity that happens that can indicate how long the visitor viewed that page. I could load one page of your site and then leave for the day, or I could just close it as soon as the page has loaded. The browser does not send information back to your site about closing or leaving your page.
The only way to determine visitor length of visit is by having 2 or more page loads.