Change the fluid every two years as a minimum. It makes a big difference.
And as Robbie says, remove the drums and check inside the rubbers. Annually is good. If not, when you change the brake fluid at two years.
My Roadster cylinders lasted almost 16 years doing this.
http://www.talkmorgan.com/ubbthreads.php/topics/612261/rear-wheel-cylinders-btr-axle#Post612261
