Most commuters make the mistake of thinking suspension forks are maintenance free. Below is an exerpt from SRAM's recommended maintenaince for basic shocks like the Dart 3 in question. Follow them and the shock will last a reasonable amount of time. For the average commuter - there's absolutely nothing convenient about the added maintenaince and is another reason I don't recommend them for casual city use by non-mechanically inclined riders.
clean dirt and debris from upper tubes - after every ride
inspect upper tubes for scratches-after every ride
lubricate dust seals and upper tubes -after every ride
check front suspension fasteners for proper torque -every 25 hours
clean and lubricate remote lockout cable and housing - every 25 hours
clean and lubricate coil spring assembly (coil forks only) -every 100 hours