You can keep a vaporizer on for as long as you like, but monitoring the air's moisture level is important. The humidity level can create an ideal mold and mildew growth environment if the humidity level gets too high. In small portable vaporizers, most will shut off automatically when the reservoir is empty. In larger portable and whole-house vaporizers, set a timer or use a humidistat that automatically turns the unit off when the desired humidity level is reached.