I'm aware many people advocate high doses, "mega-doses" even, of Vit C. However, there is a school of thought that high doses ought to be avoided. I believe this has something to do with kidney stones or something similar. I'm sure James advocates no more than 500mg of Vit C daily as such. Needless to say, some people do argue that you "can over do vit c."
I'd not use more than 500mg daily IMO.