P/S fluid continually cycles through the system in a big loop. Sucking out the reservoir and refilling will get a good amount of fresh fluid in. I would say do it every 30K. If the fluid is old/dirty, do it during oil changes 2-3 times in a row.
Brake fluid is different. It doesn't circulate. It does churn a bit, and will get circulated over time, but it takes awhile. For what its worth, the Acura factory recommended replacement of brake fluid is every 3 years or 45K miles, whichever comes first.
Forget about the "I never changed em and my car has 17 million problem free miles" type answers. Changing p/s fluid isnt as critical, but changing the brake fluid is. Brake fluid is hydroscopic, meaning it absorbs water over time, and corrodes the internals of the system. Also, water boils and turns to vapor, which can lead to brake fade. Another good reason to change the brake fluid is that it gets dirty over time, and there are a lot of little orifices and valves inside the ABS modulator. If they're gummed up with junk, the ABS is not going to work to its full potential.
When I was going to tech school one of my teachers made a good point. "The two most important systems on a car are STEERING and BRAKES. So change the fluid once in awhile."