Here's some guideline info. from the folks at US Carburetion ...
The short version is that one KW requires about two horsepower (under load)
while those two horsepower require about 20,000 BTU per hour
while propane contains about 92,000 BTU per gallon
and a gallon of propane weighs about 4.2 lbs.
So, a 7 kw generator ought to need about 14 hp. at full load, using about 140,000 btu per hour or a little over 1.5 gallons per hour or a little over 6 pounds per hour, if my math is right.
I've never run my generator long enough on propane to run a tank dry, so no real-world experience to relate, and these guidelines are the best I can do to help.