How much would be a vitamin c overdose?

No such thing unless: In general vitamin c is extremely safe and non-toxic and you can not overdose, in the sense of taking a quantity that would damage your health. There is a limit to how much you can absorb, and when this amount is exceeded you won't absorb any more and it will act as a laxative.The exception is people who have a hereditary g6pd deficiency, in which high doses of IV vit c can cause hemolytic anemia.