I am responding to you question because I just started Epsom Salt baths this week, slthough I do not know the answer to your question. I started them because I found at that my digestion is not absorbing magnesium, and the lack of magnesium is causing insomnia. So the bath is a good way to get magnesium back in and so far it's helping. Plus I am detoxing at the same time.
I am doing it every day although starting slow and upping the amount gradually. My first bath was with two cups of Epsom Salt which was way too much and made my insomnia worse. Now I am using a half a cup in the bath and have started sleeping. I will try to work up to two cups.