In humid city's in the USA and other countries do most homes have air conditioners?

I live in Africa and not that many homes have airconditioners. It has got me wondering why seeing that most of the year my city is hot. It is also very humid.

I am curious to know if air conditioning is common in other hot countries homes. If they are or not, is it because they are expensive to run? How long do you leave them on for. Do you leave them on over night?