The element has a very broad amount of current it must carry based on the wire cross-section, the total overall length, the number of parallel paths, the voltage being applied and the amount of heat lost to the surrounding materials. Assuming the only variable that changes is temperature, it would still have to be designed to carry the maximum allowable current when its -40°F to bring it up to 79° regardless of whether it’s set point is 80° or 90°. The temperature of the steering wheel element is probably 20-30° hotter than the set point upon starting to warm the wheel and gradually tapers off as it approaches the set point. Taking all that into account, the number of heating cycles and the environmental operating conditions of the steering wheel will have a far greater and more direct impact on its longevity than the amount of current carried through the heating element.