Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years. Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high. Electronic testers and test strips are commercially available to measure moisture content, however moisture test strips were taken off the market because they absorb moisture in the air before they can be used. The corrosion inhibitors also degrade over time. Degraded inhibitors cause corrosion in the braking system. Taken from Wikipedia.
Ringroad Rescue Darlington test all vehicles over two years in age for the boiling point of brake fluid using the latest electronic brake fluid testing equipment & will always advise customers to change the full braking system fluid when the test indicates it is necessary, Failure to change the brake fluid in the braking system will reduce the braking efficiency of the vehicle & also cause the braking system to start rusting from the inside which in turn we have found causes premature failure of brake calipers due to the sliders inside the calipers being impeded from moving freely due to rusting occurring on the normally shiny precision smoothed surfaces of the brake caliper components, This then stops the brake calipers sliding freely back away from the brake discs & hold the brake pads against the disc whilst driving, resulting in faster brake pad & disc wear & increasing fuel consumption.