ONE POUND of propane will produce 22k BTUs...
So it isnt really the SIZE of the burner that matters... the larger the burner, the more you have it throttled back. Now if you run a larger burner wide open the entire time, your poor little tank will be dry in no time.
I think that most people see about 4-5 brew sessions out of a single 20lb tank, but have a full spare on hand for pete sake.
This one has a 55,000 BTU burner, so you could run it WIDE OPEN for about 6.5 hours! This is because a 20lb tank holds about 16.8 pounds of actual propane.