I think you have the right idea @Paranorman in what you are saying, you don’t necessarily need to actually flush anything through your soil.
The term flushing has come to be a general term that has come about to describe starving your plant of nutrients towards the end of the flowering period – to get the plant to use up all the nutrients stored in its tissues. And this reduction of available nutrients at the end does not necessarily need to be actually “flushed” out.
And as far as how much water and how long before harvest, this will be dependent on how much nutrients are in the soil and how much are in the plant. A very large plant with lots of healthy large fan leaves will have lots of stored nutrients in those leaves. And obviously in a deep water culture setup, where the hydro water and the root zone are one in the same, no actual “flush” needs to be done, except maybe to flush any leftover nutrient water out during a reservoir change, and the last fill up with plain pH’ed water only, maybe up to a week before, but often with as little as only a couple of days before harvest as there is nothing really stored in the media at the rootzone.
I think the confusion comes from the fact that “flushing” is also used to describe the actuall soil flushing that is sometimes required from something building up and getting too far out of wack in soil, like extreme pH problems, nutrient mineral salt toxicity build ups, and general over fertilization. Although they are both done the same way in some soil grows, they are for very different reasons.
Happy growing,
MacG