Theory:
Brake fluid is used to actuate the brakes, but it also lubricates the whole system and protects it from rusting inside.
Water and dirt enter the system over time (seals are not perfect).
Water lowers the boiling point and can allow rust to build up, while dirt can make the calipers not move freely enough.
Water and dirt are probably not helping the seals last longer either.
Practice:
I've seen both
DOT fluid, and
mineral hydraulic oil brakes work fine after years of non-servicing.
I've also often replaced fluid in brakes that started working poorly after years of "negligence," and they worked fine for years after the fluid change (so, there was probably no permanent damage due to non-maintenance).
It is a fact that very ugly & dirty looking fluid usually comes out of such brakes.
Conclusion?
Based on my knowledge & experience, I recommend to most people to stick with good old rim brakes or "at least" use mechanical disc brakes.
But, for those who insist on hydraulic disc brakes, I would still say that
once a year is a reasonable servicing schedule that one can easily stick to (every two years is OK, unless you mix up if you do it on odd or even years
).
Can you replace the fluid less frequently?
Yes, and it will most probably be fine - just don't blame me in case of any problems (that are really not likely, at least in my experience - no sarcasm).
P.S.
Brake performance can degrade gradually over time.
Some people whose brakes "are fine," notice the difference once the fulid is replaced (and any air removed, of course).
Relja