Whipped butter should not be left out of the refrigerator longer than a few hours.
If the whipped butter is left out longer than a few hours it can grow bacteria and go bad and make you sick.
Always refrigerate any leftover whipped butter if not using it within a few hours.
Other butters can usually be safely left out at room temperature for up to 2 days but any longer than that it can become bad and grow bacteria.
Some people leave butter out at room temperature for a day or so to help soften the butter and that generally is okay.
Just make sure the butter is enclosed in a container or something that seals it so that maggots cannot get into the butter.