Yes...but that typically is a 5-10 minute wait depending upon if the engine is at ambient air temp and of course what the ambient air temp is. It works great for showering at the end of a dusty day on the trail as I just use the residual heat of the engine shortly after engine shut-down. Or for more gallons of heated water I'll let the engine idle.
For immediate hot water, for instance for camp kitchen type usage, a Coleman type instant/on-demand water heater might be better suited just because its quicker if your engine is cold apart of the Helton system. But then again those units are fairly bulky...