The duration of an air conditioning (AC) cycle can vary based on various factors, including the size of the space being cooled, the efficiency of the AC unit, insulation levels, and the desired temperature. However, a 40-minute on and 5-minute off cycle for an AC unit to maintain a 75°F temperature when the outside temperature is 78°F and humidity is at 45% could be considered within the normal range.
When the outside temperature is close to the desired indoor temperature, the AC unit may cycle on and off more frequently to maintain the set temperature. The duration of the on and off cycles can also depend on the cooling capacity of the AC unit and its ability to quickly cool the space.
However, it's worth noting that this is a general observation and can vary depending on specific circumstances. If you have concerns about the performance of your AC unit, it's advisable to consult with a professional HVAC technician who can evaluate your system and ensure it is operating optimally. They can provide more accurate information based on the specific characteristics of your AC unit and the conditions in your home.