You could go with the factory recommendation on change interval (though I don't know what it is) or change it when you do (or get) a brake fluid flush, should be done about every 2-3 years. If you have a stick the pedal should feel better with a brake fluid flush (both use the same reservoir) and it's just as easy to bleed/flush as the calipers.

I flushed and filled the PS fluid (ATF) on my 246,000 mile ZX2 just a couple of weeks ago and what came out was pitch black. The steering had a little bit of a "weird feel" and kind of a flat spot to it, both symptoms are gone now.