A portable air conditioner will run all day without damage. However, it is highly advised against leaving your AC on all day. This is very poor practice at energy consumption because your portable ac unit will consume an obscene amount of energy. If you want your room to maintain the same cool temperature, it is a better idea to invest in a portable ac that is designed to do so with energy efficiency. A good example is a unit that is fitted with a programmable thermostat. So your ac will run until the room cools to that set temperature, then the unit switches off. When the temperature begins to rise, the unit will come on and cool the room to the set temperature again. This cycle will continue throughout the day, thus, saving your energy a lot more.