You should replace the brake fluid in your VW Jetta every 1-2 years, or sooner if the fluid is dirty or low. Brake fluid degrades over time and can cause the brakes to function less effectively, producing noises and increasing stopping distance. Low or contaminated brake fluid may also cause the brake pedal to feel soft or the ABS light to illuminate. It's recommended to check the brake fluid during regular oil changes and replace it if any issues are detected.