There are two factors that ultimately dictate how long your LP tank will last.
The first factor is the ambient conditions when you are operating your burner. The humidity and temperature will have an influence on how long the LP tank will last. As the ambient temperature drops, it becomes more difficult for the LP tank to keep up with the demand for fuel going to the burner. This is because the process that converts liquid propane to usable propane vapor slows down as temperature decreases. Similarly, as ambient humidity drops that same process slows down because the tank is less able to insulate itself with condensation as it converts liquid propane to propane vapor.
Flame height setting is the other factor that affects how long an LP tank will last. When the control valve is set to “High” the burner will use more fuel than when the same burner is set to operate on “Low”. See chart that outlines the estimated burn time of a 20 lb. tank when considering flame setting.