I typically do mine every three to five years. I don’t use any chemicals, I simply drain and refill it, although I have been known to flush with a hose, which I believe is actually unnecessary.
@barkydog, when you’re replacing the blinker fluid every year, take the time to see if you need to refill the smoke reservoir for your electrical system. Also, if you drive a Jaguar or MG make sure that you use only Lucas High Performance Smoke.