Vitamin C is given to bowel tolerance....too much can cause diarrhea. It is a water-soluble vitamin, so any excess is excreted (although there are some who feel this is not true). Start out with small amounts and build up. Dogs do manufacture their own Vitamin C, but not always enough to overcome the pollutants and toxins they come into contact with daily.
As for exact amount, I'm really not sure. We give powdered Vitamin C, 1/8 tsp. unless the dog is stressed (then we up it to 1/4 tsp.) which is about 600 mg, I think.