Mercedes recommends (for what seems like decades) changing brake fluid every 2 years; regardless of the mileage driven. This makes more sense knowing that brake fluid is extremely hydrophilic over time (versus mileage), which means it absorbs water very easily. The absorption of water, unfortunately, reduces the hydraulic capability of brake fluid.
In a quasi-sealed brake fluid system, air (which does contain varying levels of moisture) ‘contaminates’ the brake fluid. Over the course of 2 years, most brake systems have absorbed enough water vapor to change the hydraulic characteristics of the fluid a significant and perceivable amount. Whether your car is stationary or driving 30,000 miles a year, the brake fluid is absorbing moisture through the tiny brake fluid cap vent port. I know it sounds crazy, but true.
I am pretty anal about changing the brake fluid in my Mercedes every 2 years, and I do sense a stiffer brake pedal after each fluid change; not as ‘spongy’ or weak. Can you change it more often? Of course, but I do not find it necessary. Smart seems to have crafted their periodic maintenance on a mileage schedule, while Mercedes uses both mileage and time duration. I haven't owned my Smart cars long enough to reach the 2 year mark, so my experience is only on Mercedes.
Cheers,
Jeff
2010 Smart 
1976 Rover Mini
2003 Chevrolet Corvette Z06
2007 Mercedes-Benz E320 Bluetec (SOLD)
2009 smart Passion (Gone)
2011 smart Passion
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