from what i can tell its best to flush around 14 days before harvest, if your growing in soil then it says to flush with 3 times as much water as the size of the pot, so a 3gallon pot would need 9 gallons of water, this seems a lot, but i have also been told that you dont need to flush with a soil grow, so instead of flushing you just stop giving nutes for the last 14 days of growth, this will then make the plant use up everything in the soil and will get it ready for harvest,
so taking both these ideas into practice i plan on doing a small flush so the run off runs clean, then just leave it at that, its mainly with hydro that the flush is important, it just depends on the nutes in the soil, so ill do a small flush, my plants are in 3 gallon pots, so ill place them in the sink and fill a few jugs and keep watering till ive got plenty of run off, then ill leave it at least 14 days before harvest, doing this should flush out the most of the nutes but still leave a small amount in the soil for the plant to use up before harvest.
when i harvested my last lot of plants none of the leaves turned yellow so this points to the fact that the soil had to much nutes in, so this time ill give them a flush and leave them 14 days before harvest or until i can actually see the leaves turning yellow due to lack of N in teh soil