How often? either none, or once depending on your definition. Flushing is for soil, its the soil that is getting flushed. Its to lower the nutrient content of the soil. But with DWC all you do to lower the nutrient content is add water. Some people lower the ppm near the end of flowering, and others empty the reservoir and replace with empty water. I guess you could call that a flush, but you're not really flushing anything because there's nothing there to flush