A/C cleaning should be done annually.

However I have mine inspected first before cleaning. At the place I go to, my A/C mechanic mentioned that as long as the A/C still blows strong and cools well, cleaning can be skipped.

I guess having a DIY 3M Filtrete A/C filter helps extend my cleaning intervals too. My mechanic also pointed out that the filter material helps guard against dust accumulation in the evaporator - although it does come at some detriment to overall blower power (the filter weakens the blower output).