Typically, in our climate, you should have your vehicle's brake fluid replaced at least every two years, regardless of mileage. This is also the service interval recommended by Porsche, Audi, BMW, VW and Mini.
Because brake fluid is hygroscopic (attracts moisture), over time water is drawn into the brake hydraulic system. In addition to lowering the fluid's boiling point, water will also cause corrosion problems, but to-date there are no accurate service tests for water content in brake hydraulic systems. The real indicator of brake fluid quality is not water, but copper. Corrosion inhibitors in brake fluid reduce the breakdown of materials in the hydraulic system. As these inhibitors wear out due to age, water infiltration and high heat in the braking system, copper in the brake hydraulic lines starts corrosion. When enough copper ions are present in the brake fluid they act as oxidizers and begin to attack other parts such as ABS valves, master cylinders and calipers. The amount of dissolved copper in brake fluid (measured in parts per million) is the best indicator of its quality; 200ppm is generally considered the limit of brake fluid life.
It's also important to understand that the color of the fluid is not an indicator of fluid quality. Some brake fluids naturally change color from light to dark amber as they come into contact with brake system rubber components. It's possible to find vehicles with dark brake fluid that is still good and vehicles with light amber fluid that is of poor quality.