as a general principle, the time it requires to flush the medium can be seen as 2 weeks for soil, 1 week for soilless like coco and peat, and about 2 to 3 days for hydro.. what you're doing there is not flushing the plant, but flushing the medium, so it won't have any further additional nutrients, and force the plant to use up the nutrients it has stored in itself already.. with hydro this is done within a few minutes with a water-change of the nutrient solution, while the other mediums hold on to them a bit more..
this way there will be the least amount of un-metabolized nutrients within the plant, which you wouldn't want
given you grow in soil, i would keep an eye on the trichomes.. this way you know when she will be ready for harvest.
if most of them have gone milky, i would start with the flush, so that you can harvest when they're all milky and some are amber.. the difference in trichome color makes a difference in high.. more milky means more head-high, while more amber means more of a body-high.. i personally prefer a 5 to 10% amber ratio, but that's different per person..
using something like flawless finish, helps break up and remove the salt-buildups..
happy harvest!
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