Because I never know the mineral content of any water, no matter where I am, if a house has been vacant for several months I will always run the water until I feel like at least the capacity of the heater has been reached. This is done by running all hot water faucets.
Then I reignite the heater and go do other stuff until it has a chance to reheat. Then I test as usual. I do the same thing if I smell sulfur when I first run the water.
Related to vacant houses and water, for those who may have forgotten, here's the safety article about why else you should run substantial hot water at vacant houses.
eLCOSH : The Dishwaser Exploded