There has been no formal testing on that but there are a few data points that have come up.
With severely ill people (terminal illness) exposed to around 1 mg chlorite per kg body weight it takes 4 - 5 months before the depletion of essential minerals and hormones takes its toll and they become sicker. People who are healthier and have a good diet should be able to go longer.
To put this into perspective you need to know your body weight in kg. Take your weight in pounds and divide by 2.2 to find that value. Next you need to know how much chlorite you are exposing your body to. To determine this you need to know the concentration of chlorite you are using and how much you are using.
MMS is 28% sodium chlorite by weight and is made from 80% pure sodium chlorite. This gives you 22.4% sodium chlorite. Each ml of Miracle-Mineral-Supplement exposes you to 22.4 mg chlorite. Since Miracle-Mineral-Supplement is usually measured in drops and since drops vary from dropper to dropper and person to person you need to run a test to see how many drops you form per ml.
Once you know the amount of chlorite you are taking and your weight in kg you simply divide one by the other.
At the other end of the spectrum we have been using chlorite in drinking water for animals (and people) and after a little over 6 years of 0.25 mg per kg chlorite exposure along with a healthy Diet we have not been able to measure any adverse effects. Given a good diet and lots of exercise and loving care we expect our animals to remain healthy and they have. One thing we have noticed is that our water buckets remain slime free. That means that the quality of water remains high.
While the levels of chemicals we are using are higher than what is normally allowed for water treatment but at these higher levels they effectively deal with the microsystin problem that the people of Ohio are currently experiencing in Toledo. I believe that eliminating toxins from the drinking water helps keep the critters healthy.