water to waste with plain pH corrected water when you flush. When water is pouring out the bottom you're there (not flooding like Pakistan, just coming out of the drain holes steadily). At day 1 of week 7 of flowering I planned on stopping all nutrients and watering with plain water as I normally would (water to waste). If you mean for flushing because of nutrient burn and that sort of thing as a repair step mid-grow then use 2-3x the volume of your container for 2 days then return to your normal water/feed schedule when the plants show improvement. Don't over water.
That is why I grow aero-hydro. When my trichomes show all cloudy w/50% amber, I flush my roots for 5-7 days with fresh water everyday, dumping the water, and replenishing it daily. My bud taste like bud, not like potting soil or nutes. I had some bud that was an indoor potting soil grow a year ago, man it had a nasty, nasty taste to it, what made me decide to grow aero-hydro instead of potting soil. I know other potting soil grows do taste very good, this one didnot, probably didnot flush the roots. I flush my roots weekly for 24 hours after I do my weekly Dump/Flush. It prevents nute lock-out, root-rot, but mostly my gals thrive from it, once they get their new batch of Flora Bloom, Flora Micro and alittle Calmag+, they just explode w/growth, smell and look awesome and are very healthy..