Flush once? Twice? Three times???

Ok so I have a dwc setup and have now reached the preharvest phase where I have to flush! My problem is that I don't know how much times I have to change the water in the reservoir during that week or two! I would really appreciate the help what ph is best for that water??

I personally change the water twice a week with straight Ro water at ph level of 6 for 2-3 weeks before i harvest

Thanks man I'll end up doing that! And if I use a flushing agent such as clearex by botincare would it help the time so I would only have to flush for two weeks??