Keeping it simple
Every day cars: Cleanse the car at least twice a year (daily driven cars get more embedded dirt than most), wax a minimum of every 2 months especially in the warmer summer months.
Occasional use cars that live in a garage: Cleanse once a year, wax every 3 to 4 months, remembering to show it more love when it gets warmer.
Garage and show specials: Cleanse once a year, or cleanse and wax before each show or once a year to keep it looking show-car special.
The average car: stored under "normal" (Is there such a thing?) conditions, would benefit greatly from being waxed every second month.
Be honest about the action your car sees and from the information above find the balance that matches! Simple huh?