It is like MPG on a car, only with far more variables.
I don't use my burner for anything other than boiling the wort. Some use theirs to heat their strike/sparge water too. I like to leave the burner on low/medium since otherwise the flames just come up the side of my pots. A lot of flame will boil faster and harder, but it also doesn't allow me to control the initial hot break foam, and I would get boilovers more often with the gas all the way up.
Weather is also a huge factor. If it is windy, you'll use more gas. If it is cold, you'll use more gas.
With all that being said, I have never run my tanks out while brewing, but I have filled up somewhere in the 2.5G range on a tank after 4 brews this season. I believe I could easily stretch 6 or 7 brews with a tank, even with spring/fall/winter brewing.