I filled the HLT up with 28 gallons of 53 degree water and it took about an hour to get to 170.
The boil was more of a rolling boil. However if I were to insulate the BK, I'd be able to get something more vigorous. I don't plan on doing large brews like that all the time and really would only do 25 gallons at most. So for a 25 gallon boil. I'm sure it would have been fairly vigorous.