I have always considered this a wishy-washy number based on a lot of assumptions - do you guys have a links to documentation discussing this metric? I need some backup for how I feel about this metric.
No hard stats, but I completely agree that it is a rather meaningles stat. One site we work on shows about 30% single access sessions, an average of 5 page views per visit and average session length of more than 12 minutes. Median session length is a better stat, but even that is questionable. A site like Ebay ought to be the benchmark (where it would seem people spend lots of time) and scale down from there.