Here's my long answer.....
I use 20lb (typical gas grill size) tanks have been getting a consistent 5 1/2 60 min boil sessions per tank. I have a 15 gal kettle that has a larger 18" dia. so I need to boil 7.75 gal to end up with 5.5 gal. I do all-grain brewing so my wort temp going into the kettle is ~165F. I start the boil process with the kettle covered until a boil is achieved. Most times my kettle is boiling in ~15 minutes then I turn down the gas supply to keep a rolling boil (more than a simmer but short of vigorous boil).
This has been winter brewing (coldest session ~10F, warmest ~25F) in a garage with overhead and rear passage doors wide open. I try to position the burner to limit wind exposure while keeping it ~5ft from any combustables in the garage. Once the weather warms up I hope the gas will last for another 1.5 sessions.
BTW - I have 2 propane tanks so I don't have to run to the store to get more when one expires. I have a CO detector with PPM display I test regualrly with my car exhaust to ensure it is working.