How many days of flush until yellowing?

The older leaves going yellow is not a certainty, and not a pre-requisite for harvesting. If the leaves have not already started yellowing, this indicates a rich nutrient solution has prevented this yellowing from occurring and it is not a problem. Giving her cold water is rubbish, in half an hour the water would be back at "room temperature", so is unlikely to "trick" the plant into doing anything other than giving the plant a shock. Unless the water was chilled constantly, there is no point in giving cold water, besides this will just slow the plants metabolism and delay maturation and possibly also stress the plant by having cold roots and warm foliage. It will also not stimulate trichome production, the trichomes main function is to protect the seed ovules in the calyxes from over heating, so simulating autumnal conditions would trigger a reduction in trichome growth due to the plants slowed metabolism indicating that summer has passed, hence warm/hot weather has also passed, negating the need for enhanced trichome growth. Flushing is another internet "fad" with it not being a hard and fast rule for every type of growing method. Prolonged flushing will indeed starve your plants and delay ripening, which then lengthens the flush which then delays ripening, which then lengthens the flush which then....... I think you get the picture. If growing hydroponically, as in DWC, 3-4 days of plain water just before harvest, would be adequate for "cleansing" the plant and maybe extending this to 7-10 days if growing in soil or coir, would also be more than adequate. So continue with your routine and there is no need to have any concerns about your plants' leaves staying green and healthy. If you are still more than a week away from maturity, I would suggest giving your plant some half strength nutrients so she can maintain a healthy metabolism and not be starving for nutrients while she finishes flowering and then re-flush with plain water from 4 days before harvest. Since all nutrients in DWC are dissolved and in a suspension, means that as soon as the solution is replaced with plain water, all fertilizer salts are also removed, whereas when growing in soil or coir, the dissolved nutrients are bound to the soil, meaning the plant has longer access to nutrition and therefore also requires a longer "flush" before harvest. Hope this helps,......... Organoman.