Purge clean my bike brakes once a year, needed or not. Brake fluid is cheap, the task is easy, and fresh fluid will keep the lines and seals pliable and healthy. Friggin ester based fluid can draw moisture through stainless steel. Aircraft with phosphate ester based hydraulic fluid (the EVIL Skydrol 500
have their fluid changed out every 3 years, and many commercial operators require an annual sample hit the lab so they can monitor content. I hate the shit, and think they should all use Dawn dish liquid, since it has been proven to work! LOL.