I would say it would take a lot more than 15 minutes - more like 30 or longer. I have often been surprised how long it takes my Yanmar
(30GMH - 3-cylinder) to even become warm to the touch on the outside of the engine block - starting from dead cold. When you think about it, the block and all the coolant
around it first has to heat up enough to even open the thermostat at all, only then will warm water start to flow thru the water heater - and it will be fairly 'cool' coolant
to begin with ... When the thermostat does open, it first has to mix in all the coolant that is in the pipes and inside the heat-exchanger/water heater, and all of this has to also warm up to HOT before it finally starts to have any effect on your six gallons of cold water. Try taking six gallons in a monster cooking
pot and put it on the stove .. it takes a mighty long time for that much water to get really hot. The reason is the 'Heat Capacity' of water. Water has a very large heat capacity. It can absorb a lot of energy without affecting its temperature very much. This makes water a very good medium as cooling
Let's all move to the Tropix - and use SOLAR!