A lot of confusion comes about because flushing has two meanings. It means to just feed you plant plain water for the last 7-10 days so the plant "flushes" the chemical nutrients (fertilizers) out of its system. Trust me, you don't want to smoke chemicaly tasting weed, I have and it's vile. I'll call this a pre-harvest flush.
The second meaning of flushing comes from "flushing" the plant with a lot of plain, non-clorinated water. How much water you use depends on why you are doing this.
One reason to do this is to remove excessive salt build up from the soil that develops over time just from using fertilizers. In this case you would want to run about from 1 to 2 times the soil volume, basically till the water runs nice and clear. This is a light flush that I'll call a maintenence flush.
Another reason to do this is if you have burned your plant by feeding it too much fertilizer. In this case you would want to run anywhere from 3 to 5 times the soil volume, depending on how badly you over fertilized. So if you have your plant in a 3 gallon container then you would flush it with 9 to 15 gallons of water. This is a heavy flush that I'll call an emergency flush.
So, to answer your question you can do this the last 7-10 days before harvest. Start this cycle at the normal watering time; that's why there's the 7-10 days leeway.
1) Do not use any more fertilizers. You may use a carb/food like molasses.
2) (Optional) At the first watering of plain water run 2 times the soil volume through the pot. Do this very slowly. You do not want the soil to compact, but you do want the water to completely drench the soil. Look for the water to be crystal clear. If it's already clear you're golden, go ahead and give the plant the the water, just make sure it drains off and doesn't leave the soil soaked. Just like normal. If the water is still cloudy after 2 times the soil volume then go ahead and keep doing it till the water runs clear.
Hope that helped.
