Usually every year in all of my cars, but I won't let it go more than 2 yrs. It also depends where you live (i.e., heat, moisture from rainfall/fog, etc.). I inherited a Toyota w/ 93K mi on it 5 yrs ago, and the brake system had internally rusted due to the moisture in the fluid. The vehicle spent its entire life in So. CA, garaged, too. The rear cylinders finally started leaking, master still worked, but had this sludge at the bottom of the resevoir. I'm amazed it didn't fail. I basically had to replace everything.
Thanks guys for your comments. Since mine is a used vehicle and I don't know, I am changing mine. It did live in S. AZ which is a zero humidity enviroment, but like your other advise I have changed out all of the other fluids in my new tome LC and this is the last one to change. What I don't understand is why Toyota doesn't have this change as part of their regular maintance routine. I've noticed over the years that dealers don't even recommend changing the brake fluid. Most owners will never even think about changing their brake fluid. Just kind of weird.