I don't think there is a specific amount that would be considered overdose. If you are having negative symptoms, don't do any more! If you don't feel well, just stop doing it. Sometimes it's hard to tell. Be careful, you are young, and bath salts are a serious drug, containing serious chemicals. Do some research first. And if you must do it, keep the doses small. From what I know of bath salts, a smaller dose will have the same effect as a larger dose. Be careful.