This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: Brake fluid should be changed AT LEAST every two years. Bleeding should be
done yearly and after an Autocross. Brake fluid absorbs water which
corrodes the lines. Bleeding regularly will significantly increase safety
and life of your brake components.
DOT 4 fluids have a higher boiling point than DOT 3 fluid but are also more
hydrophilic and therefore must be replaced more frequently than DOT 3.
 
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