Time on Site - Metric from GA

Time on site is calculated as the time spent between the first pageview of a visit and the last pageview of a visit.  If your visitors are only visiting one page on your site, then there is no second pageview for that visit for GA to calculate an average time on site.  In those cases, GA assigns a time on site of 0 seconds because there is no way to calculate how long a visitor spent on that page.

If you have only one page of your site tagged, or your site consists of only one page, it is likely that most of your visits will only have one pageview during their visit and have an avg. time on site of 0 seconds.

Also, the time on site is calculated by subtracting the time stamp of the final page view from the time stamp of the first page view.  Therefore if a visitor only views one page, the time on site is 00:00:00\

---Goolge Analytics