Leaving a propane heater on for an extended period of time, such as all day, can be potentially dangerous if not done properly. Propane heaters require proper ventilation to ensure that the byproducts of combustion, such as carbon monoxide, are properly expelled from the living space. Inadequate ventilation can lead to a buildup of toxic gases, which can be harmful or even deadly if inhaled. Additionally, propane heaters should be properly maintained to ensure that they are functioning correctly, with no leaks or other issues that could pose a safety risk. If you plan to leave your propane heater on for an extended period of time, it is important to make sure that the heater is located in a well-ventilated area, that it is functioning properly, and that it is regularly maintained. If you have any concerns or are unsure about the safety of your propane heater, it is recommended that you consult a professional.