If car is not doing heavy duty racing then heck once a year would be just fine.
If doing lots of hard braking then i'd check few times a year.

I tested both my 1999 car and pickup, pickup I have never changed the brake fluid and it still tested just fine.

Vette I may have changed brake fluid once for brakes and twice for clutch and it also tests out just fine.

Point is the cyberspace fad for vettes is somehow if you change the fluid for clutch every month that somehow it will prevent clutch problems is bullshit and only causes people to open cap of master often and allowing water vapor getting in and that is what kills the fluid.