My Brupaks boiler took about 1 hour from cold for approx 5.5 Imperial Gallons.
Test Driving Blighty?
Don't forget you're going to be going from sparge temps rather than cold.
Build yourself a heatstick or two- Here's how:www.cedarcreeknetworks.com/heatstick.htm
Whether added to your stove or propane burner, it really speeds up boiling time.