How do I know the exact Google Analytics time-on-site for one page website?

I have a one page website but a lot will happen on that page over time by means of Ajax. According to GA's rules, no matter how long the first page of the webpage is opened, without further page visits, that visit is marked as "Bounced" visit and average time-on-site will be zero.

How do I know the exact time of users' visit on site(that page)? Can pol开发者_运维百科ling behind the scene be a solution?

Looks like polling behind the scenes is the answer. Although, it won't be exact.

The guys at Google Analytics suggested setting a timer that fires off an event at a set interval. You can then use these events to calculate time on site for your single page site. See this video (skip to 27:22): http://analytics.blogspot.com/2011/03/web-analytics-tv-17.html