My propane fire pit manual said it's about 8 hours on a full 20lb tank at full flame.
I just turn it up until the flame is going good and that's only about 2 full turns on the little knob thingy.. It will turn about 10 times more, but does not produce any more flame, so I just stick to the couple of turns..
Lasts me several hours during the evenings for a long weekend trip, using it every night I'm running it, but I do think it is about 8 hours of total time on a cylinder, as I also use it for the Coleman 2 burner stove and after a long weekend, it's almost empty.. I go thru one of those little screw in propane things pretty fast on just the camp stove.. I'd think it would last maybe and hour or less on the fire pit??
But, I do like it when we have our burn bans! I have people coming over during those times and wondering how I have a "fire" going and I just point to the propane tank and the pit and they back off thinking I was actually burning wood... ha, ha...
Mitch