It has become a matter of conventional wisdom that insurgencies last an average of 10 years and that the insurgents win about 40 percent of the time. These statistics have appeared in USA Today, PBS, Pentagon media briefings and on National Public Radio. The insight these numbers are meant to convey is that counterinsurgencies are inherently long and difficult struggles against wily and resilient foes, so it is unrealistic to expect rapid, quantifiable progress in the near term. Fortunately, these statistics are misleading and the associated analysis is wrong.

http://www.armedforcesjournal.com/2008/05/3434645