You really don't need to flush them, You can do some research online, But from what I was told when I asked on on another site, they said flushing is for toilets, something about nutes being absorbed and not in there chemical form any longer, I dunno I still will if i feel The need to. if you do my best time was 7 days prior to flushing taking down, I would do 3 times the pot size two times before the final day. I hope that helps you out.
I give my plants a final feeding about a month before harvest. After a month of pure water, they are fully flushed.
I flush the last 2 weeks before harvest....and like someone mentioned, most of the nutes are broke down by the time u harvest, but it doesn't happen overnight....if u feed ur plants up until u chop it, then u will certainly have the chemical taste to it....a week would be enough time to flush, but I always err on the side of caution