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:

This metric measure the cumulated time spent on pages (in seconds)

Formula : Sum of inter-page duration - (bounces pages duration + last pages of the visit duration).

You will find below an example of this calculation:

This metric measure the average time spent on a page per page load (in seconds).

Formula: Cumulated duration of a X page / (Loads of this X page - loads without duration)

The last pages of the visit are excluded as the pages of bounce visits.

You will find below a detailed example of the calculation:

As the last page of the visit is excluded, the calculation is the following one, 478 / 6.