In CoCo I go at least 4-14 days depending on the strain and how I feel and how much I fed the plant in the first place, in DWC, I will run water for last 3 days, no longer. I find when I cut CoCo in 4 days flush, not much difference then the 14 days, the only time it really makes a difference is if I use molasses, then flushing gets rid of that molasses burnt taste that can happen sometimes. now I just use "Sweet" in place of molasses.