As with every other fluid in your car, brake fluid contains a main ingredient along with protective additives. The main ingredient in brake fluid doesn't degrade, but the additives do. The most important additive is a corrosion inhibitor. The corrosion inhibitor prevents internal rusting of steel brake lines, calipers and ABS (antilock brake system) components. Once the inhibitors are exhausted, even minute amounts of moisture can cause dangerous and costly corrosion.
Most car manufacturers recommend a complete brake fluid flush every two years or 24,000 miles. Rather than rely on a general recommendation, you can test the actual condition of your brake fluid. Remove the cover of the master cylinder and dip a strip into the fluid. Shake off the excess fluid and wait 60 seconds before comparing the color of the strip with the guide on the package. The guide tells you when it's time to change the fluid. Find a package of 25 BrakeStrip test strips at brakebleeder.com.