Taken from Cannabis Culture (put as much faith in it as you want too)
"Much time and thought has been put into the feeding needs of each part of marijuana's life cycle, yet for some reason the final stages of resin development always seem to be ignored. But the vegetative period of plant growth is only setting the platform for us to produce the trichomes that we are after.
Flushing in particular seems to be something that is over-emphasized by many of today's growers. Many growers "flush" their plants with straight water or clearing agents during the final weeks before harvest in an effort to improve taste and smokeability. The theory is that this forces the plant to use up stored nutrients that may affect these qualities. Although this is certainly true to some extent, what many are forgetting is that not all nutrients can be moved within the plant.
Nitrogen, which is the main factor in poor-tasting bud, can be moved within the plant. If not present in the root zone a plant will take it from the older leaves to support newer growth. Calcium, however, is a nutrient that cannot be moved within the plant, if it is not present in the root zone it is not available for growth. Little research has been done on nutritional requirements of cannabis during the final stages of flowering, but it seems likely that calcium is vital as it is crucial in cell division. A calcium deficiency at later stages could therefore adversely affect trichome production.
This is not as serious of a concern for soil-based growers, as lime or other calcium sources which are mixed into the soil likely will provide sufficient nutrition even while flushing with pure water. But hydroponic growers using very pure water sources with little naturally occurring calcium could have problems. Flushing is certainly a valid technique, but is easily overdone and is not a quick fix for overfeeding earlier in the flower stage."
I only flush for 4 days or so, the longest ive ever done it is a week. I also use AN's final flush (which could play a role, though honestly I doubt its that big of a difference). Never had a chemical taste in my weed after a proper cure.