I don't know how much different it is with hydro vs soil. Soil can build up dissolved salts much easier and in larger amounts I would imagine than with hydro. But maybe it's different for DWC?
I would look up something like a DWC guide to flushing, or look for some discussions on it. Like I've said before, hydro isn't my game!
That being said, in soil, I usually flush 7-14 days before chop.. I've found it's better to flush a bit later rather than a bit early. Like, my feeling is that you can always just give them a few extra days, peak ripeness is a 1-3 week window. If you flush 3-4 days too late, and your concerned, you can always just wait out the harvest a few more days. However, if you flush early, and the plant just doesn't get ripe, in addition to not being ripe, your plants are now stressed from not having any food.
A lot of experienced people who have been growing for a long time
(NOT me
, I just read what they say and regurgitate) will tell you the last few weeks of flowering are extremely vital, and the plant will need food the most then to pack on the extra weight that comes on in the last week. Then you have a lot of inexperienced people swearing by stuff like 3 day dark periods, and long flushing periods for a "smooth smoke". But most of the people grow some pretty awesome looking buds, so it's hard to see who's right without the science, that we don't have.

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With flushing, my goal is to start to discolor the leaves like a week before I chop. I figure the plant will loose all food as salts in the rootzone, then it will start going for the leaves, and it will begin sucking all the nutrients out of the foliage (that you will be smoking).. Once quite a few leaves fall off, and the leaves are discoloring all over I know the plant has used up the food in the rootzone, and is now munching on reserves in the foliage. The 'reserves' of nutrient inside the foliage, is what people claim to "harsh" their smoke. But then like I said, people like uncle ben will say they haven't flushed a plant in a long time and taste no harshness in their smoke.
And to note, I flowered for 72 days on both plants.