Brake Fluids have a limited life, not only because of water absorption but because corrosion inhibitors and stabilizers are depleted over time. Wear particles and rubber fragments will also slowly build up.
Most automotive professionals agree that glycol based brake fluids, (DOT 3 brake fluid, DOT 4 brake fluid, DOT 5.1 brake fluid) should be flushed, or changed, every 1-2 years. Many manufacturers also require periodic fluid changes to ensure reliability and safety.
Electronic testers and test strips are commercially available to control brake fluid conditions measuring moisture content.