Jim,
A pound of liquid propane contains 91,500 btu. And 5hus a full 5 pound tank could contain 5x 91,500 =457,500 btu's. AThus, for example if your Firebowl consumed 18,000 btu per hour a 5 pound tank could theoretically run it almost 25 hours. In reality you probably never have a completely full propane tank.
This info also handy for knowing how long your heater could run, if you know it's btuh rating.
There you go