Every year as part of our "over the top" maintenance schedule. I essentially get a good look at each termination and give it a tug to be sure it is tight. According to David Smead of Ample Power fame the typical 1-2-both switch is a fire waiting to happen and it always bothers me that I can't inspect them.... One of these days I am going to exchange ours for the Hella type he recommends....
Bruce