Propane contains...
~4.2 pounds/gallon
~91,000 btus/gallon
~0.23 gallons/pound
~21,000 btus/pound
A 17lb tank would contain around 357,000 btus of propane.
A 20lb tank would contain around 420,000 btus of propane.
At 42,000 btus/hour: 17lb is 8.5 hours; 20lb is 10 hours.
(1 pound of propane would last 0.5 hours)