The more you need them the hotter they get. Water turns to steam if it gets hot enough. Hydraulic fluid is incompressible, steam is not. The possibility exists that you might be coming down a long hill and have to use the brakes a lot. The pedal could go to the floor as the steam in the brake lines compresses under pressure.
Brake fluid is relatively cheap and painless to change.
If done annually the hydraulic components would last almost indefinitely. Use good D.O.T.4 brake fluid from sealed cans. If your car has French hydraulics, changing the fluid biannually is also a good idea. Use only genuine Citroen LHM fluid and keep some spare liters on hand, only a very few sell it. Distilled water is cheap.