I've heard that a high-powered burner (like a Kick-a-Banjo 210K BTU) will chew through a 20lb propane tank in about 5 batches.
I would assume that a turkey fryer (usually around 60K BTU) will last a couple more, but since the KAB isn't going to be running at full power, don't expect to triple your time-to-empty.
PS I use a 10 gallon aluminum pot, no issues yet. It'd probably be 2-3x as heavy if it were SS, and I would've paid twice as much.
Triple Cat Brewery
Primary: Summer RyePA
Tertiary: Strawberry Blond
On Tap: NB's Oatmeal Stout, Eagle Mills Hard Cider, Hefeweizen, Black Lager, Warsteiner Dunkel Clone
"Bravery is not a function of fire power." ~J.C. Denton