How do you know when to start to flush a auto flower?
I have never planted auto flower, so this may not be as effective. I do not flush my plants unless there is an issue I am trying to solve. That said, I start to water with only RO (<15ppm) water when I feel I am 3 weeks or so from first harvest. I have found this allows the plant the opportunity to use up any nutrients in the soil as they slowly dissipate with each watering, it also allows the plant to use up the stored nutrients in her roots to make up the difference. I feel this helps with taste and creates a cleaner harvest.
Note, keep an eye on her when you start this, each strain reacts a bit different. You will see the effect and can judge if any adjustments are needed.