Gary,

As Colin said, just limited by amount of propane and maintenance needs 
of unit.

My brother had a 15K standby generator  installed  after last summers 
storm, along with propane tank (or tanks).  In this recent storm, it ran 
the entire house  4 full days until power came back.   He did have the 
propane guys out once to top off the propane, but that was really due to 
not knowing just how long it would be until power was restored, rather 
than any fear of running out.

73, Steve KW3A