If your car is usually parked in a garage or otherwise sheltered from the elements, waxing it twice a year should be enough to protect the paint finish. But if your car is frequently exposed to snow, rain, road salt or even just spends most of the time outdoors, it probably should be waxed every three to four months.
Wax not only gives your car a shiny glow, it adds an extra layer of protection against harsh weather, salt, bird droppings, tree sap, ultraviolet rays, vehicular and industrial pollution, and other assorted crud found in the air and on roads. By not waxing your car on a regular basis, you are giving these corrosive substances a better chance of damaging the clear coat, the finish on top that seals the paint, and exposing the paint below. In addition, if you have to rub and scrub an unwaxed car to get it clean, you risk rubbing through the clear coat and into the paint.