it depends on the car. On some cars I've owned (Honda, Nissan and Volvo), I changed it at 100K, just because. On my Ford, I flushed it at 30K, then again at 60K, because the pump was making a ton of noise. After the second flush, I now do a partial swap (suck out at the reservoir and add fluid) every oil change, and that seems to keep things happy. This car is hard on PS fluid...not sure why, but it seems common to Fords.
So, I guess "as often as needed" or "every 100K", whichever comes first...