I have a 45k propane for ~1,400 sqft with mostly 12' ceiling, 10' for a 3rd. R21 walls, R30 ceiling, set also 50 degrees 24/7, runs 68 or so when working in there.
When less than say 20 degrees outside it takes ~90 minutes to get to temp but it can certainly get it hotter if necessary. My heater...