How frequently do aircraft get struck by lightning?
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.