Weekly washing is best because some contaminants quickly do serious long term damage to paint if they are allowed to remain. Two such organic compounds are bug remains and bird droppings. These bio-hazards contain complex proteins that bond to the surface and organic acids that penetrate and break down the surface, eating into your clear coat if not removed promptly. Weak spots in the clear coat make your vehicle susceptible to corrosion and discoloration.
Another problem contaminant is brake dust. It contains metal shavings from the rotors and adhesives used in the production of brake pads. Try to picture the dirty plume of brake dust and roadway chemicals that travels along with a moving vehicle.