In 13.5 the Time Spent Per Visit report listed the average time spent in the total in addition to broken out by the various time buckets. This disappeared in 14. Currently the only way to get this is via the ExcelClient
I got asked by our CEO: How is average time on site trending? Up, down, the same? I have no easy way to answer this in Version 15 without running the report for each month and adding the numbers to an XLS and making my own graph. Lame.