You want to have the plants to be in all water so they use up the nutes in their system and clear the flavor and chemicals out of there system. Like nay other hydro you want to flush for at least a week give or take for personal preference.
Great post, was going to ask this myself as I plan to harvest in about 2 weeks. I figured 7 days was enough but I had heard as little as 3 days was enough.
I plan on running 10 days as well for flush I'm at 33 days of 50-55 so this will be my last reservoir change tomorrow before using R/O water and some florakleen