Flushing, or the correct term leeching, is rinsing the GROW MEDIUM in HYDROPONICS systems, of excess nutrient buildup. It only needs to be done a couple times, generally going into flower to reduce buildup of high N fertilizers and once about a week before harvest. Otherwise feed normally, only reducing nutrient levels the least week or so, as the plant requires.
Soil grows, generally, don't need flushing unless heavily supplemented with chemical fertilizers, and even then its not always needed.
Flushing is a marijuana specific MYTH. No where in agriculture is it done.