Off the top of my head Id say about 3-5 times the size of your pot.....8 gallons. x 3....25 gals of water.....pretty quickly yes......that's the point of flushing. All the water also MUST BE ph correct......between 6.2-6.8 and after flushing it should come out the same from your soil as whatever ph you poured in the top.

With the photos of your plants from 10 days I can now see how quickly they've gone downhill so I think they are WAYYYYYY overnuted as well as abnormal ph. I'd get to flushing them as soon as possible if you can.

The link may give you other ideas to try but I have a pretty good feeling you are in for some work.

Good luck.