Maybe it's the long day I've had, but when I see average time on site is that per session / visit or per visitor? I'm pretty sure it is avg. time per visit / session, but I wanted to make sure and the Google help docs mostly use the term "visitor".
Interesting. So, if I wanted to know (loosely) how much time on total was spent on my site during a day by all my visitors, would I multiply average time on site by # of unique visitors or # of visits? Example: Yesterday I have 50 visitors and 100 visits with avg. time on site of 4 mins. Were 200 or 400 minutes total spent on my site that day?
no. Average time on site means that all of your 50 visitors may spend either 4 min max or less then 4 min (0.00.00) as its on and average of spend on site by total visitor. The average time on site is calculated by dividing the total time on site for all visits by the total number of visits. This includes bounces so in your example would be 4min/2 users.
I think more than anything it is a good relative measurement to compare historical data and data between my own sites and other sites in the same vertical / niche.