The rule of thumb answer is an evaporative air conditioner can reduce the temperature by much as 14^oC depending upon the external air temperature and humidity level.

Your home’s orientation, shading, cross ventilation and insulation will impact on how hot your house gets.

To maximise the benefit of evaporative air conditioning, the evaporative air conditioner needs to be sized correctly to treat your home  and must use good quality clean pads, ideally Celdek, to work most efficiently. 

High humidity negates the benefits of having evaporative air conditioning as the air drawn into your home already has a high water content and cannot draw up the cool water on the filter pads.  That’s why evaporative air conditioners are popular in Perth, Adelaide, Melborne and Sydney but are not used in Brisane or Darwin.

Most people are comfortable when the indoor temperature is up to 26^oC. As a rule of thumb, evaporative air conditioners are effective for people living in a dry climate in modern homes with R2.5 and R3 insulation for temperatures up to 38^oC. For temperatures above that, the indoor temperature will creep up to 30^oC on a 42^oC day with low humidity. 

Note, strictly we should refer to evaporative coolers not evaporative air conditioners. Air conditioners use refrigerant to cool the air.

Want to understand how evaporative coolers work, check out this American video.