The 21 day rule also includes flushing time if you want to flush.
Internet will break at this point and people will explode with opinions. I PERSONALLY believe you should flush as I notice a difference in smoothness, burnability, taste and smell. However there are people who believe flushing is for toilets and not plants, the debate rages on.
If you're interested. I flush for 10-14 days in soil allowing 3-4 days of dry out time before harvest. Remember youre flushing not drowning. Little and often is key.
Coco I flush for 5-7 days with a flushing agent for the first flush and water for remaining time. Again allow for 2-4 days dry out time.
Hydro I flush again with a flushing agent for the first 24-36 hours and flush for 3-5 days allowing only a day drying time before harvest. Changing res daily after flushing agent.
All water is not pH'd as it doesn't affect nutrients being expelled via the roots and the plant will absorb water regardless.
I use cold tap water for all parts of my flush and have been known to chill with ice bottles for an hour's or so before watering. Some shock is good as I forces extra THC production as a last ditch effort.
Before harvest also I leave them in complete darkness for 24-36 hours with the fans still running. Again small amount of additional shock to promote production!