Denny is right in saying it depends on how much you use the brakes. Back when we actually used the vehicles, we flushed the system annually. Sometimes that wasn't often enough. Water causes rust. Rust makes a rough spot in the wheel cylinder which causes a leak. Pulling a hub on a 3/4 or larger isn't much fun. Flushing the system is easier. I have gotten lax because I don't drive any one of my jalopies much. But I would think that most people who have only one classic vehicle would maintain them. With a pressure bleeder, its not even as dirty a job as changing oil.
