Generally speaking yes, it is safe to leave your AC unit running all day. Since summer days can have you exhausted and you just need the cool air to keep yourself from melting away. Therefore, leaving the cooling unit on the whole day will not cause any harm or disaster. But it could be a problem if your whole neighborhood does the same and leave their AC on as well. This could cause your AC to trip or your neighborhood could possibly have a power outage. A possible solution to this could be when you know your neighbor has their AC high, try simply turning up the temperature to lessen the burden on the power grid. But also be informed if you use the AC unit the whole day you are likely to get a higher electricity bill.