Average Time Spent 00:00:00

Time spent is a calculation of time stamps.. (Page B timestamp - Page A timestamp to get time spent on Page A)

If Page A is the only page in the visit or the last page in the visit, there will be no time spent... Bounces is "a single hit per visit", last pages in the visit (with earlier hits in that visit) will not count as a bounce.

There may be some calculation error when trying to do this against an unspecified value such as a campaign? 

You can test various scenarios by going to your site yourself and use a custom tracking code for each scenario. Then create a visit based segment using your special tracking code for the scenario you are testing. You know what you did, and can see how it comes out in reporting.

At the risk of over communicating, Average Time Spent on Site is calculated using sequences as its denominator. A sequence is a contiguous series of “same” values. So if you saw two of the same values in a row, it would show that as 1 sequence. It gets even more interesting for the Campaign variable, which is basically an a evar.

Time is tracked across persisted values in evars. In the following example, a tracking code is set on the second page of the visit. The user sees 2 more pages before the user returns to the site using a different tracking code. The user then sees a page and returns to the site again using the original tracking code.

For the tracking code 123, the first sequence is 3 minutes. The sequence is then interrupted when the second tracking code is set. The last two pages create a second sequence for the tracking code 123. Time is not tracked for the last page of that second sequence because it was the exit page so the second sequence lasts 1 min. The Average Time Spent on value for tracking code 123 is 3 min + 1 min / 2 sequences, or 2 min.

Not sure about the technical reason, but I always find 'avg. time' to be zero for 'unspecified' values when using evars or classifications. I'm suspecting it has something to be with a value not persisting, and there is no second element to make that calculation.