It depends on how far the unit is from the faucet being used. When we lived in Hong Kong, we had 3 small units-one under the kitchen sink and the other two in the two bathrooms. It took about 7 seconds for hot water. But if you turned off the tap and then turned it on again, it would take another 7 seconds for the hot water to resume.
It also depends on whether you are on well water (not recommended due to silt and sand) or city water.