What is the average number of flight hours that US private pilots require before passing their checkride?
This document from and published by the FAA does include a range although they do not cite their source or where they pulled the data from. It is as official a number as I can find. 


  One difference between a part 141 school and a part 61 school is that
  fewer flight hours are required to qualify for a pilot certificate in
  a part 141 certificated school. The requirement for a private pilot
  certificate is 40 hours in a part 61 school and 35 hours in a part 141
  certificated school. This difference may be insignificant for a
  private pilot certificate because the national average indicates most
  pilots require 60 to 75 hours of flight training.


The range most likely varies heavily due to the way people train. I know people who have gotten it in &lt;50 and people who are up around 90 and still have not flown the check ride. It varies largely with how much time you have to train as well as local weather. I was flying once a week (occasionally twice) during my training) and I was somewhere around 63 when I got my ticket.