There is a "Days Since Last Visit" metric... it's not so much time based.. just number of days.. that is about the closest you will get without a custom solution....
While there are ways to track "time" between start and end events / actions, those have to take place within the same visit, so they wouldn't be relevant to time between sessions.
The only real time based solution I can think of would be to use Raw Data Feeds and SQL to calculate the average time spent between the last visit and the next using Visit Num and the timestamps of the last and first hits... but if you don't already have Raw Data feeds being processed, this is a large project to undertake, and because Raw Data isn't processed in the same way as what you see in Workspace, it means you have to apply logic to make it in line with your existing reports.....
