flushing is not necessary. if you think it is overnuted or something, maybe it has a benefit? just like you, plant's have cell differentiation. Also, think about outside grows. Do they leech the ground of all nutes, or is that even economically viable relative to how deeply you'd have to flush the earth below. That typical tastes fine and can by top shelf in good climates.
when you start to consistently see bits of amber in most places, you probably ahve 5-14 days. Some finish faster than others. Soil can flush for ~14 days. I'd cut that in half for coco. and hydro i think is 24-48 hours tops.