If you have ABS, I recommend every 2 years or 24,000 miles, whichever occurs first. Without ABS, I recommend 3 every 3 years/36K miles.
I think you will find Ford's maintenance recommendations to be similar.
The reason to change it is brake fluid aborbs moisture over time, and moisture is bad for three reasons:
1. corrosion of the caliper piston/cylinder microfinish.
2. corrosion of the small passages in the ABS control unit
3. water can boil off during hard repeated braking, reducing brake effectiveness, because water vapor is compressible, while brake fluid is not.
Your owners manual will specify if your systyem is designed to use DOT3 or DOT4 brake fluid.