Time Spent

This module tracks, by Ajax, how long a registered user took to read a page and the amount of time he spent on the entire site.

You can configure which content type and role should be tracked and view the report, exhibiting the total hours (or minutes/seconds) a user spent reading a page and at the entire site.

This module just track node pages and registered users. Taxonomy or views pages aren't tracked, but can be if the code is edited.
If you need to track anonymous visits, consider using Google Analytics (GA can't track registered users by its privacy terms).

There isn't any module dependency.

USE CASE:
You need to track and exhibit a statistic report with the amount of time a registered user took reading a page and the total time spent by him at your site.

BENCHMARK
A fresh install will have each ajax call varying about 110-130 milliseconds.

SPEEDING UP:
In Drupal 6, if you install High-performance JavaScript callback handler, everything will be faster. Install it and clean cache. Time Spent is already pre configured to work with this module. With this integration, a test install will have each ajax call varying about 30-40 milliseconds.