How long do you flush a dwc

When you are changing out your water from a dwc how long do you let the roots flush and do you have to change the water after the flush some of my leaves are turning yellow some would like a rusty color and I’m in life are starting my third week of flowering

I simply change res since I have several units my plants are only attached to the lid I simply swap whole res out filled and ready to go I add h2o2 at this point anything left on the roots is washed into new solution and the old res is dumped washed and ready for my next change. I also tend to run h2o2 every second day which keeps any bacteria down in my system