By chemical nature, Brake Fluid is Hygroscopic..... that is to say... it takes on water, or absorbs it from the air. The less you open the brake fluid reservoir, the less likely it is to absorb that moisture. Most Brake systems are closed sytems, and are not vented to the atmosphere nowadays, thus reducing the need to "Change" it so much.... however, in a moist climate, or a constantly changing situation (From hot to cold and back again from traveling in and out of those types of climates), it certainly is a good idea to change it even more often than the 2 years or 100k miles mentioned above. If you don't drive like you stole it all the time, the factory brakes will take you a VERY long way before needing much any service. Of course an occasional situation will arise and some may last longer than others.... yeah.... YMMV.