to see how long it takes for boiled water to cool down to somewhere
between 165 and 185 degrees F, which is considered the "sweet spot" for brewing green tea. (Since water boils at 212 degrees, you can see that this sweet spot is considerably cooler than just-boiled water, which will "cook" the green tea and alter its natural flavor and aroma.)
Although the exact amount of time to wait after the water reaches a boil depends on the amount of water in the cup or pot, I would say wait at least 5 minutes. To speed cooling, you can remove the kettle lid after boiling.
Remember, we want to coax the flavor out of the tea, not cook it!