It should all be flushed annually. Brake fluid absorbs water right out of the air.
Nothing you can do to stop it, replacing the fluid with fresh material from a seal container is the only way to control it. This is also the case with autos.
Failure to replace the fluid will eventually result in holes rusting through the brake lines at their lowest point (gravity will collect the water into droplets there)... it that's a line, it will burst. If it's a brake caliper cylinder, the seal will fail.
Far better to swap the fluid in advance. It may take 2,3,4,5 years for it to get bad enough to cause failure; you are sure to avoid an issue by catching it before it has any chance of doing damage.