As it is already available for the pages report or for the prop reports to add the Average Time Spent as a metric, it would be very nice to have this feature also for the Campaigns reports.
I agree that this should be a metric available to campaigns or any other conversion report out-of-the-box, but there is a work-around. eVars, including campaign, can be related to events. Events can be used to either count instances, increment by a numeric value, or increment by a currency value. If you set up a numeric incrementor event (increment by a non-currency value) you could capture "time spent" in that event. Essentially, for each new page view you would increase the value of this event by the number of minutes (or seconds) that elapsed between the previous image request and this image request. This metric could now be viewed for each eVar.
1-If a campaign is set on the first visit, but the campaign variable expires in a week, the total time spent would be not only the first visit time, but also an subsequent visits. You can control this by only sending the "time spent" event data on campaign visits.
2-Single page visits would skew the data. In the interface, visits with only a single page view aren't counted in the totals, but they would be with this solution. You could build a segment that excluded one page visits which could offset this issue.
3-The final page view is never counted in the time spent on site metric. This is true in both the interface and with this work-around.
Seriously. This metric should be in all Traffic Source reports!!