Brake fluid should be replaced after every race meeting really! It all depends on how many laps you do and how much heat you put into it.
In the Mini, I run Motul DOT5.1 and RB14 pads (Formula Ford Spec) - with the cars low mass, I don't have a problem with pad fade or fluid boiling etc...
In the WRX I used to run standard fluid and RB74 pads - never had fade with them either, but I never strung more than 4 laps of Sandown or Calder together.
Some people seem to be much harder on brakes and tyres than others, and often it doesn't mean better laptimes. There's more time to be gained from taking correct lines maximising corner speed than braking later and getting a poorer line.