I don't know how strong your nutrients are or even what they consist of sorry. What I can say is that you don't need to flush at all unless you have a toxic issue. If your feeding strength is not too strong and the plants have healthy green leaves, not too dark not yellowing, I wouldn't change much tbh. By week 7 you could half the amount of strength but use the same watering volume just to ensure their are no last minute build ups as the plant lowers N demand etc (if you knew it was an 8 week strain). There are more complicated ways to do it but half strength for the last week is an easy way to look at it when your leaves are a nice green, maybe with slight fading even.
I can't help you with when the last week is, this depends on your strain but can also depend on your preference and even environment. Certain total environments are heavily speculated to reduce flower time.
All I feel safe to suggest is try to keep the leaves a nice looking green for as long as possible, give the plants 3/4 strength nutes in what you ''think'' will be the last 2 weeks. This will be a happy medium unless you specifically know flower duration.
To get a greater handle on this you will need to read up on
elemental ppm and work out what your nutrient line consists of. You will then have to find info on what ppm value MJ likes. It's a long process but a worth while one. You will be completely winging it otherwise.. and likely to be going off of the advice of bro science growers.
This page provides a common language spoken by all fertilizer solutions and a way to mix them for growing cannabis hydroponically. Sometimes referred to as the Lucas formula, this page explains the true origin of this well known proven formula and the project that led to its discovery. Emphasis...
www.angelfire.com
This link won't make much sense to you right now, but once you understand elemental ppm it will. I don't know how scientifically accurate it is as of recent date but it has certainly served me well in being in the right ball park. I use half the amount of nutes I use to use directly because of this chart with no effect on plant health, so that's half the nutrient money saved and half the chance of running into toxicity issues in mid/late flower.
I am no expert on toxicity or even under feeding. All I know is when I follow this type of guide in the link I never see issues of discolouration. In other words I am not an expert in it because I don't need to be ;p. Keep root zone temps in line and you will also avoid having to be an expert in leaf colours as that creates imbalances too.