The time it takes for a propane tank to run out once disconnected from a grill, burner, or stove unit will depend on several factors, including the size of the propane tank and how much gas is left inside.
Propane tanks come in various sizes, such as 20 lb, 30 lb, or 40 lb tanks, among others. The size of the tank will determine how much propane it can hold. Additionally, the level of gas remaining in the tank before disconnection will also affect how long it lasts.
Generally, propane tanks have a pressure relief valve that helps prevent over-pressurization when they are not connected to an appliance. As such, they will not release gas rapidly when disconnected. However, the gas will still slowly dissipate over time.
Without specific information on the tank's size and the remaining gas level, it's challenging to provide an exact time frame. But to give you a rough estimate, a typical 20 lb propane tank used for grilling might last anywhere from a few days to a couple of weeks when connected to a grill, depending on the frequency and duration of usage.
When the tank is disconnected and stored indoors, the rate of gas loss will generally be slower since it is not being used or exposed to temperature fluctuations that can affect pressure. Nonetheless, it is always recommended to store propane tanks in a well-ventilated area and outdoors if possible, rather than indoors, as a safety precaution.