mmm... I love Green Crack... one of my favorite varieties!
but, no again....
You have hit on one of my favorite subjects lately... the perversion of the word, "flush."
what exactly is your goal of this flushing that you speak of?
flushing to a cannabis growing expert means that you have flushed the soil, by moving 3x the container size in water through the soil so as to cleanse it of all salts and debris. A flush is a one and done thing... you don't start flushing on one day and have it continue for some time and many days after that. A flush is usually done on a normal watering day so that as far as the plant is concerned, it just got a good watering, and then life goes on.
Are you maybe confusing the term flush with the act of starving your plant to death at the end by giving only water, somehow thinking that you are cleansing the plant and making it smooth to smoke, but instead only stunting the plant, right when it could be and should be growing the fastest that it ever had during the entire grow up until then? If so, you are not alone... many people these days seem to be similarly confused.
If we are indeed talking about a proper flush of the soil, then yes, the proper time to do it is around week 7, right before the last 2 weeks of bud swell has started. Timing it for this day cleans the salts out of the soil that have been building up all during bloom, assuring that the roots are no longer salt restricted, so that you can achieve full uptake of water and nutes during this last critical phase of the grow. Having unleashed the full potential of these roots to provide what is needed, then hit your plants with bloom nutes as well as finishing nutes such as Terpinator, to finish out the buds strongly, letting them double in size and weight while also giving them the full flavor potential of the strain and the maximum size and weight that you have been preparing the entire grow to be able to achieve.
Sadly because of the bad information out there that is so prevalent these days online, a lot of people are doing this all wrong. I hope that having seen these words, I can keep you from also going down this path. Do it wrong, and you will never know... the plants are very forgiving and they will adapt, and you will be spared from knowing the potential you might have achieved. Take a leap of faith and trust the science and you will be rewarded... you may never know how much, but everyone will love what you have produced.
You may have heard them say that we now produce pot that is many times stronger than what was produced in the old days... it is techniques like this that have made this possible... we now know how to grow a lot better than we ever did before and we have learned that many of us followed some old bad advice. Live and learn they say, and always keep an ear to the ground for a better way.