you need to experiment with bowel tolerance. excess is excreted in the urine, so there is no danger of toxicity.
start them out with 500 mg daily and slowly build up to 1000mg. if diarrhea occurs, cut back a little to the last dose that didn't affect stools. at times of stress you can increase the dosage, the body will really benefit from it. just make sure to build up slowly, or have some pumpkin on hand, since large doses of vitamin C are a natural laxative.
make sure you use a product that is made with ester C and bioflavonoids, they work together for maximum benefit.