Switches should really be changed every time the batteries are changed. They are the most fragile part of the electronics system and the most susceptible to vibration damage. I cycle test the batteries every 6 months and change them every 3 years assuming they haven't failed a cycle test. A fellow club member changes his switches and batteries every year because he's had a failure before in the second year. But he flies usually twice a week, so he puts more use on them than I do.