How much propane is used per hour to run a 2.5 kw Onan generator??

depends on engine size, RPM it's running at, how much load is on the engine, etc etc. I suggest, weighing your empty propane bottle, filling it with 5 pounds of propane and then run it and full load, full throttle and run it till she dies then figure it out from there.
You don't even have to do that.  Weigh the propane container before running the machine, then weigh it after you shut it off while keeping track of how long you ran the generator for.