As a general rule, it is recommended to change the brake fluid once a year or every two years. Of course, this also depends strongly on the way of driving and the mileage.
The reason for this measure is that brake fluid is very hydrophilic, that means it tends to absorb water. When applying the brake, the braking system, and with it the brake fluid, is heated up in such a way that the water starts to boil and form vapour bubbles. These bubbles are compressed during braking. Therefore, the increasing proportion of water in the system reduces the brake pressure. The specialist garage is able to determine the boiling point of the brake fluid and thus state when it become necessary to change it.
If you are in doubt, better change the brake fluid once too often which is good for safety. In addition, changing the brake fluid with the Stahlbus Bleeder Valve will cost you considerably less money than usually because the consumption of new brake fluid is very low as the system is absolutely tight.