there is no black and white answer to this because there are various factors that affect the usage of propane in a smoker ranging from temperature to all the way towards weather conditions. however , as a rule of thumb , you can expect a 20 lb propane tank to last for around 18 to 20 hours under given conditions. although this time duration can increase or decrease depending on the various factors . among these factors , temperature at which the grill is the most determinant. for example if you propane smoker is running at 170 degrees at a sunny day , then the propane tank will last longer than if the same smoker was running at 200 degrees in the same weather conditions.