How should I track time spent per page?

Several metrics can be used to obtain such data, the usefulness of which will vary depending on the circumstances:

• Time spent/ pages: this gives you the average time spent per page for all pages in all entering visits with page loads (bounces and entering visits with no duration are excluded).
  
  Here's how it's calculated: Cumulative time spent / (Page views - Bounces).
  
  
• Time spent/ pages (page views): this gives you the average time spent on a specific page for all loads of this page (bounces and exit pages are excluded).
  
  Here's how it's calculated: Cumulative time spent on a page P / Loads of page P (with duration).
  
  
• Time spent/ pages (visits): this gives you the average time spent on a specific page for all visits that have viewed the page.
  
  Here's how it's calculated: Cumulative time spent on a page P / Entering visits containing page P.