Ideally, we believe that the best air conditioners for grow tents and grow rooms should be left on all the time that the sources of heat are on. This is because they help counter the level of heat and a temperature equilibrium is maintained. However, different growers have different techniques of growing their marijuana and they may want the temperatures cooler or hotter based on their specific needs. However, make sure you do not turn it on for so little a time that heat actually becomes a problem or you turn it on for so long that the plants start to feel the effect of this cold weather which slows down their growth.