I've actually never replaced my brake fluid until I had a complete brake job done or had things such as the brake caliper or brake calipers or brake lines changed.
Most manufactures recommend a brake fluid change every 20,000 miles or every 2 years but I don't change it that often and have had no problems.
Chevrolet recommends changing the brake fluid every 45,000 miles but even that to me seems excessive but that's just my opinion.