Well, it depends. Some manufacturers include it in their maintenance schedules and others don't. Mercedes-Benz, for example, says brake fluid should be replaced every two years or 20,000 miles, and Porsche Towson says that should be done on most Porsche models every two years regardless of mileage. On the other hand, most Chevrolets can go 150,000 miles or 10 years, according to Chevy's maintenance schedule, and many Ford, Chrysler and Toyota vehicles don't even list brake fluid as a regular maintenance item. Check your car's owner's manual to see what the manufacturer recommends. You might also want to discuss the subject of brake fluid with a trusted mechanic if the manufacturer doesn't give any guidance. Don't be surprised if a mechanic suggests replacing the brake fluid periodically, because mechanics probably have seen what can happen if you don't. What can happen? Even though brake fluid dwells in a sealed system it still can absorb moisture over time, and that can lead to corrosion in the brake system. Moisture also lowers the boiling temperature of brake fluid, and this can reduce braking effectiveness in repeated hard stops.  This is why Porsche and Mercedes recommend that brake fluid should be replaced every two years. Replacing brake fluid is cheaper than replacing brake lines or a master cylinder that has corroded.