I'd estimate about 55,000 BTU/hr which is well within the range of many propane turkey fryer burners.
8.35 lb/gal * 8 gal = 66.8 Lbs
Assume the water is at room temperature ~70 F and needs to make it to 212 F (boiling at sea level).
That's 142F change of 66.8 lbs. of water.
1 BTU is the amount of energy to increase 1 lb. of water by 1 degree F.
142F * 66.8 lbs = 9486 BTU in 0.5 hr, so 18971 BTU/hr
But, if I remember correctly, the typical efficiency of heat transfer for a propane burner is about 35%,
So, you'd need about 54203 BTU/hr.