There's mixed reviews on flushing, to be honest. But, most say it depends on if you're growing organic or synthetic. A lot of synthetic nutrient fans flush for two weeks. I grow organic, so i don't really need to flush. I still do flush for a week and a half, just to be safe, though.
Do a little research on it and see what you think. I'm a fan of being safe instead of sorry. But, everyone is different.
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I hope this helps!