The dealerships are great for adding stuff to ordinary service calls. Keep in mind that the tech gets paid by the task, the service writer gets paid on commission, and the owner needs to make a payment on his new yacht.
When your brake fluid needs replacement depends...much on the usual humidity in the air in your region. It's more about humidity and time than it is about mileage.
Moisture in brake fluid is often ignored. There was a fatal collision at a state park near us a few years ago. It was a very steep hill, and the minivan driver didn't know to downshift. He used is brakes to the point where they overheated, the moisture in the brake fluid flashed to steam, his pedal went to the floor, and the fatal crash resulted. The same thing happened at a national park in the next county, the RV was coming down a long steep hill, that driver didn't know to downshift either, a passenger reported after the crash that she heard the brake pedal hitting the floorboards hard as the driver tried to brake until they crashed. No major injuries on this one. A friend was towing his horse trailer down a steep mountain on the other side of the state. Suddenly he had no brakes. He was able to stop by downshifting and using the parking brake, and the road flattened out. After he got stopped and caught his breath his brakes had cooled enough for him to carefully drive home. His mechanic diagnosed just old moist brake fluid.
Also, if anyone greatly overheats their brakes, not only do the pads & shoes & rotors & drums & all the rubber parts need to be replaced, but the brake fluid got cooked and needs replacement also.