Based on a survey of large US legal departments, the average number of years a general counsel occupied that position was 4.6 years. The median was even shorter, a mere three years. This data comes from a survey during 2007 of Fortune 500 legal departments and is reported in a paper presented by Prof. Michele Beardslee at the Georgetown Conference on the Future of Law Firms at 19.
The tenures seem too short. It is unusual for general counsel of large companies to leave other than by retirement so less than five years in the job strikes me as too short (See my post of Aug. 14, 2005: suggests around seven years average tenure for GCs of major companies; and Nov. 11, 2005 on tenure.).
But who am I to argue with a statistic?