I think it really varies depending on everything from the BTU rating of burner, to the ambient temp and cooking vessel.
I'm using a 10k btu Bayou Classic burner. I use an 8 gal. aluminum pot. I have been brewing in warmer temps of 75ºF plus.
I figured after 4 brews my tank needed a refill, but at the U-haul place the guy said I was only half empty. I'm sure I could have at least gone another 2 boils before getting low enough to be concerned.
I worried about it because most people here seem to need a refill after 4 sessions. I assume it's because they're using 25-50k btu burners, and many are also heating their mash and sparge water with the propane. I mash in a 5 gal. MLT rubbermaid cooler inside with water from the stovetop and boil the wort outside on the propane burner.
I may have to look into getting one of those tank level sticker things myself...
Man, that's probably more than you wanted to hear. Sorry.