Some car manufactures state it in their owners manual while some other cars don't state how often your brake fluid should be flushed out and replaced with new brake fluid.
Car manufactures such as Mercedes-Benz states that the brake fluid should be flushed and replaced every year or 20,000 miles whichever comes first.
But for other cars flushing brake fluid isn't really needed and I've never flushed out my brake fluid until I worked on the brakes such as replacing brake wheel cylinders or brake hoses etc.
That only happens every 100,000 miles or so which is often enough for me and my vehicles.
You could however flush the brake fluid and replace it every 2 to 3 years if you want to but anymore than that it really isn't needed.