There are two main factors to take into consideration:
The sensitivity you and your passengers
have to the air you breathe. Regardless of time of year and where you live, there can be particles in the air that will irritate the respiratory and/or olfactory systems in individuals. Do you drive through seasonal pollen, smog, exhaust fumes from trucks and busses, airborne dust, intermittent wet and dry days where spores and mold growth spikes? People with allergies, asthma or even just a great sense of smell will be affected to varying degrees. The elderly and the very young are more susceptible that the average person but it really all depends on your individual comfort zone, One person in the same environment can be oblivious to odors while another can suffer headaches and nausea. A good rule of thumb is at least once or twice per year depending on your comfort level. However, the ver sensitive, asthmatic or immune system suppressed individual will want to do it more often. Said from the discomfort of a foul smelling car environment, keeping the evaporator core clean keeps the ventilation system operating at peak efficiency, saves wear and tear on the compressor (the heart of the system) and minimizes fuel consumption to run the system.