[bobcowan]

Depends on what you use the car for, and what kind of brake fluid you have.

If the car is a street cruiser and doesn't see that many road miles, and you used a standard DoT4 fluid, changing every 3-4 years is more than enough. How often do you change the fluid in your DD?

If you were silly enough to use a DoT5 silicone fluid, it should be changed every 1-2 years. More if you live in a damp climate.

If your engine compartment gets hot enough to degrade brake fluid, you have a serious problem.