The regular service interval for a brake fluid flush is 2 years. This interval is accelerated for cars that see the track or racecars.
There are a couple of common reasons to flush your brake fluid regularly contamination and age. Brake fluid is hydroscopic, it absorbs water, and water saturation decreases the boiling point of the fluid. Under hard braking a tremendous amount of heat is generated from the friction of the pad and rotor contact.
This heat can cause your brake fluid to actually boil within your brake lines. When the brake fluid boils, it creates air pockets within your brake lines. This causes problems because air compresses under pressure(say from pushing down the brake pedal). This causes a mushy pedal and reduction in braking ability. Brake fluid is used in braking systems because it doesn't compress.
There are many ways for moisture to enter your brake system. Condensation from regular use, washing the vehicle and humidity are the most common. Most of this is unavoidable because the brake fluid reservoir needs to be open to the atmosphere. It has a small hole in order to avoid a vacuum being created by the brake pads wearing down.
Performance cars, or any car has been on the track, should be flushed yearly or as needed. Aside from the usual flushing, brake bleeding is generally necessary because it removes the air that builds up when the fluid boils. Stainless steel brake lines do help maintain braking while on track for extended periods of time but bleeding the system is recommended after track events.