Just as a reference point it would take ~8000 BTU to bring 6.5 gal from room temperature to a boil, assuming no losses. So a burner that's putting out 30,000 BTU/hr could do it in about 15 minutes if it was running at maximum power. Since I don't think you want to run them wide open all the time, figure 45 minutes in real life. Like jeffy said, most basic turkey fryer type burners are in the 50k BTU/hr range. That's what I use and for 5-10 gallon batches they're plenty powerful.