If you are asking how much bleach to bring your ph down, don't use bleach go buy a big bottle of ph down, it costs like 5 bucks and will last 100 gallons, with directions on the bottle for how much to add.
If your asking about the bleach for the cleaner water/roots it seems the thread has come to the consensus that bleach is not the way to go. However Hydrogen peroxide works great. I tried it, and within 2 days my roots where already turning whiter, they had been fairly slimy and brown. I am convinced, and will be using it every res change from now on. I used 1 tbl or 15 ml per gallon. But I was using the 3% house hold brand. If you are using the 35% kind it is somewhere between 2-4 ml per gallon.