Let's get focused back on technical issues rather than "gut feel"....
Here's an excerpt from a technical article about brake fluid, meassuring wet and dry boiling points etc that covers the topic well IMO...
4.0 When to Change Brake Fluid?
Many motor vehicle manufacturers recommend
checking brake fluid every 12 months, or changing
fluid every 24 months. The amount of moisture that
brake fluid absorbs depends on a number of factors
including the type of fluid, humidity, type of
brake hoses, condition of the seals and mileage.
Brake fluid will absorb 1% or more moisture per year
of service life. A two year old vehicle will have 2 to
3% water in the brake fluid. 3% water reduces the
boiling point of DOT 3 brake fluid by approximately
175°F (97°C). 3% water in DOT 4 brake fluid reduces
the boiling point by 162°F (90°C). (See Chart
next page)
Generally, the type of driving should influence when
to change brake fluid. If the vehicle is used for
towing, is driven in mountainous regions, or has an
ABS system it would be best to change the fluid
sooner. All vehicles should have the brake fluid
changed when the water exceeds the minimum wet
boil point.
Motor vehicle manufacturers may recommend
brake fluid changes at lower water content/
higher boiling point than shown on the chart. OEM
service recommendations should be followed.
BMW reccomends 3 years for first flush and 2 year intervals there after...same as Audi/Vw...both use Bosch braking components (ABS valve blocks.) so I'd bet that Bosch has given some input to the car makers here about how to keep their components in top operating shape...Interesting to note both makes switched to requiring only DOT 4 LV (low viscosity) fluid at about the same time a few years back....so I'd guess Bosch drove that change as well....
