Bronze is ideal for exterior use because of its resistance to corrosion and its ability to take a fine, aesthetically pleasing finish. However, when the protective coatings of wax are not maintained they will eventually be lost, leaving your sculpture unprotected from the influences of the environment.
For a minimum clean, you can use a soft cloth, feather duster, or soft-bristle brush. Never use a chemical cleaner or metal polish on your sculpture as doing so will damage the finish, often very severely impairing the sculpture.
For an indoor bronze maintenance once a year is recommended (Twice a year if in a humid or corrosive environment).