Is it common to have to replace the brake pads and roters when my C240 4-matic is only at 31000 miles?
brake pad and roter replacement on 2005 Mercedes-Benz C240 4MATIC
If regular maintanence is done on your brake system, you shouldn't need rotors for several brake pad changes. Glazing and warping from high speed stops, can usually be taken care of by turning the rotors. The cost of aftermarket rotors for a lot of cars today is only a few dollars more then the cost of turning them.
Most European car has a "soft metal" brake disc for a good braking performance. It's common that you need to change the disc together with the pads because they are worn down to or near the minimum specs. For some models you can find performance aftermarket brake components that last longer than the OE.