It is recommended that your brake fluid is renewed every 2 years. This is because brake fluid is hygroscopic, which means it is able to absorb water molecules from its surrounding atmosphere. Failure to refresh the fluid will result in a loss of effectiveness of the braking system, as the properties of the fluid are changed by the water content, particularly the boiling point.
Taken to an extreme, not flushing the system with some regularity, can result in a mayonnaise-like accumulation, like that seen here.
Needless to say, the brakes on this car were hugely improved by cleaning the reservoir and thoroughly flushing the system with new fluid.
