How frequently do aircraft get struck by lightning? We have several questions here about what happens to an aircraft when it gets struck by lightning (e.g. this question), and at least one answer from a former pilot who knows he was struck once and thinks he was struck other times. How frequently do aircraft get struck by lightning? 

Additional commentary on how do you know you've been struck would be appreciated, but I'll make that a separate question if it's more appropriate.
It is estimated that on average, each airplane in the U.S. commercial fleet is struck lightly by lightning more than once each year.


Edward J. Rupke, via Scientific American.


  The Federal Aviation Administration estimates that on average, lightning hits each airliner in U.S. service once a year.


Washington Post

It looks like commercial airliners get struck about once a year. I was also able to find an FAA source, but it doesn't look like it's the original source.