Yes, you can use "Average Time on Site" based on a time span.
The description is a little confusing... so I'm going to try to illustrate how Adobe records "time spent"
User Enters Site on the home page, time stamp is 1:00:00
User goes to Page A, time stamp is 1:02:35 (Adobe then calculates the time on Home page as 1:02:35 - 1:00:00, basically 2 mins 35 seconds)
User goes to Page B, time stamp is 1:03:23 (Adobe then calculates the time on Page A as 1:03:23 - 1:02:35, basically 48 seconds)
User closes the browser (there is no time stamp recorded... Page B is left without a time calculation
When you look at the pages by the Time Spent Metric, you will get the individual page times... if you don't break it down, and look at the whole site, it should show the time spent on the site as 3 mins, 23 seconds.....