I cannot tell you how soon your brakes will need replacing. Brake wear is dependent on how and where you drive. We I can tell you is that generally speaking brake pads don't last as long a they did only a few years ago. This is due to the brake pad material being used and the smaller, lighter design of the newer brake systems. Until you have a feel for how long your brake pads will last I would suggest you have the tires rotated and the brakes inspected every time you have the oil changed on your 2008 Volvo C70. Once you have determined what the normal brake pad life you may be able to perform the tire rotation and brake inspection every other oil change depending on how ofter you change you oil.