Hey,
Personally, I'd guess whatever the service schedule says do that, or you can get brake fluid testers, which as brake fluid is hygroscopic absorbs moisture, the tester measures the water content, and indicates weather you should change or or if its good to go
I think bi-annual is a bit too much, unless maybe your going to the tracks heavily, but every few years is fine, but that's one man's view!
From what I can remember, a brake servo is the vacuum part connected to the master cylinder and just helps with the operation of braking when you push on it to brake
Hope it helps