Chlorine of 5 won't affect the process. There is no way to specifically tell how long the alk lowering process takes. It comes down gradually. You may not have to add Borax to raise the ph back up after you get your alk where you want it. You may just be able to 'aerate' it back up. If it doesn't come up, then you can go ahead and add some Borax, but try the aeration first.
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