I use a high-pressure LPG Banjo with a 30PSI regulator. When raising the temp quickly, like at the start of my boil, I crank it up enough that it gets pretty loud and the flames are 3" tall (or more). I've never timed it but it doesn't take me but 15-20 minutes to raise 7 gallons of mash temp wort to boiling.
Once I achieve boil I crank it way down.
These things do suck down some propane, which is why I just got 3 low pressure versions in the mail this week for my new sculpture. Going to give those a try and see how they go.
Don't get me wrong -- I love the high-pressure version. but it has a tendency during hot days to freeze the tank over.