I don't know when (or if) my brake fluid was changed before I bought the car, which is 30 years old, so potentially an extreme case.
The system was full of rusty sludge. So far, I've had a foot-to-the floor brake failure which I attribute to boiling brake fluid. I've replaced one wheel cylinder, (the other one is weeping) and rebuilt one brake caliper. Basically the whole system needs rebuilt/replaced, and ideally I'd replace the steel brake pipes (which look OK externally), with copper, though I don't know if that'll be possible here.
I always used to leave fluid change until I had to work on the brakes, but this car has convinced me regular fluid changes are a good idea.
Its worth pointing out that, in my experience, once you get gunk in the system, flushing does NOT get it out, but early fluid change should stop it happening in the first place.
Flushing is easy. If you don't want to pay the dealer to do it, don't pay the dealer to do it. But do it.