Just guesswork, but I would say that half an ounce would be plenty over the short run.  Usually when something serious happens or is in the area, I mix our three found dogs and our found cat's water about 50/50 and they drink it up with no problem.  At other times we just put a few teaspoons in the water every once in awhile.  One of our dogs was attacked a couple of months ago by a skunk that I am afraid was rabid.  The attack was in broad daylight and the skunk went out of its way to come attack the dog, which was on a chain.  Then when I got a fallen branch off the ground and tried to seperate the two, the skunk attacked the branch - growling and biting it - and then made a beeline for me.  A  homerun swing later and the skunk's back was broken and it was lying about 15 feet away where it came to earth.  After cleaning the bites with vinegar and colloidal silver we began mixing the water 50/50 and putting digestive enzymes, skullcap (also known as maddog cap for a good reason) and olive leaf extract in the dogs food - and Luella and I took copious amounts of colloidal silver and the supplements too since we handled the dog after the bite (that is why vets don't treat dogs who have been bitten by a rabid animal - not because they can't be treated, but because the risk of getting rabies handling the dog is considered too great).  Months later, none of us are foaming at the mouth . . .

For what it's worth, I am not a fan of MMS and would not take it myself or give it to our dogs or cats.