Hi Panda, I did a DIY on how to do ATF flush for a Myvi. The process isn't too different for other Jap cars.
http://www.zerotohundred.com/newfor...e/385386-diy-flush-atf-and-change-filter.html
But it's a 2-man job so don't attempt this by yourself. Preferably 3 if you don't have experience doing this.
And not all Japanese cars use the same ATF. Some Toyota Type-IV, some Dexron III, some SPIII so don't mix them up. Type IV is Type IV, Dexron III is Dexron III. Unless it's a mutli-grade ATF don't mix them, the only exception is Dexron III and Dexron II. D3 is backward compatible with D2 but not the other way around but D2 these days is pretty rare.
Power steering is a closed system so oxidation is slower compared to the other oils and it's not exposed to heat (much). So safe would be 40k km or 60k km. But you'll need to change it sooner or later, don't wait till it has problems, it'll be more hassle than it's worth to fix it by the time it breaks down (the steering rack).