Replace around 2 to 3 years. Brake fluid absorbs moisture over a period of time. Even in a closed system moisture can get in. It can rust the delicate anti-lock components of the brake system , which are very very expensive to repair. If you are keeping your car for a long time...it is worth it.

Re your update 2 NO 'qualified' mechanic is likely to tell you that. Brake fluid is hydroscopic and absorbs moisture over a period of time. Have a look in your cars service book and it will tell you what the recommendation is from your cars' manufacturers.

Brake fluid is hygroscopic and will over time entrain water from the atmosphere, through the vent in the reservoir cap usually. Water in your fluid can lead to steam being generated in the system under the heat of braking, and this steam is compressible unlike the fluid itself. Thus you can suffer poor braking and even compete brake failure if it gets too much.

That's why it's recommended that you replace the fluid with fresh very few years or so.

The fluid absorbs water. Under extreme conditions this water content of the fluid can boil causing brake failure.

Mercedes service schedule a brake fluid change at (I think) 48000 miles.

Changing brake fluid is like changing car insurance companies. It supposed to be done every four or five years. But, nobody does it!

Every 2-3 years