The cleaning frequency of stainless steel depends on four points;
A rule of thumb for cleaning external stainless steel is when you clean the surrounding windows also clean the stainless steel.
Wash the surface of the stainless steel (both surfaces if stainless steel mesh) with soap or mild detergent and warm water. Rinse in clean cold water and dry. The guide below provides an outline of how to specify and clean external stainless steel mesh
Note: Rainwater is an effective natural cleaner, stainless steel that does not get rained upon will need more frequent cleaning.