I think a lot depends on the outside Temp. During hot weather We would NEVER leave our dogs without air conditioning during Fl.summers, the temp can /does climb considerably,lately several have died due to heat stroke in a vehicle. In cooler weather we might leave them for a little while. It is not worth taking the chance
to leave our pets in the heat. BTW: Fl. just passed a law that if an animal is inside a car in the heat and suffering a window can be lawfully broken by law enforcement and the owner can be fined as well.