Calculating Web Surfing Duration

There are various techniques and products that produce estimates of end-user web surfing activity, but I'm directing this question not towards desktop monitoring software but rather those tools, such as Webtrends or Bluecoat Reporter, that produce an estimate of activity based solely on a static proxy log or local internet history.

Is anyone aware of algorithms/products that have been defended in court? Or even better, cases that have successfully discounted attempts to put a value on an end-users web surfing?