i agree with yard. you can try to fix nute problems all the way up to a week or two from harvest. i normally allow 1.5-2 for a final flush, but, i''v heard 1 week is enough in most cases.
you can allow for less of a flush if you wean them off the nutes over time, vs going full bore right to the end. if you are slightly nute burning your plants the whole grow, i think 2 weeks is more acurate, while if they are nice and healthy with no overdosing all they way through the last few weeks, 1 week should be fine.
when you go to give your final flush, when i was in soil, if i had it in a 3 gallon pot, i would flush with 12 gallons of fresh water, its way overkill, but it ensures you flushed all you can out of the soil, and there is no second guessing.
with hydro, i just pop the res out, dump it, put it back on empty, and pour a gallon of tap water (i use RO in my res.. but for doing this i just use tap) through each of the pots to wash the pots and roots off,
dump the res, then fill the res with tap water, and gently swish the root mass in that, to get the last of the roots rinsed.
dump it again, and fill with RO water.
the tap water used for this sits out for 24 hours to get rid of chlorine, and my tap water sits at 120ppm.. (so in my eyes, good enough to grow with, but i like to take out guess work and use RO)
again, that pretty much overkill seems show most people probably just dump the res and refill with plain water.
forgot to add, with the fresh water, i add a bit of superthrive. its not a nutrient, just vitamins and hormones, i do this so i dont accidently shock them when rinsing everything.
