Recommended daily allowance (RDA) for an adult is about 90 mg/day. If you smoke, add 35 for a total of ~125 mg/day. While RDA is set based on recommendations for prevention of deficiency, it’s been shown that anything above that in a healthy person doesn’t seem to improve biomarkers of cellular oxidant stress (at least that of endogenous lipid peroxidation).

If you really feel the need to be saturated with vitamin C “just in case”, your blood level hits maximum at ~400 mg/day; anything above that is peed out, so don’t waste your time and/or money! :-)

Sources:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC55540/
http://lpi.oregonstate.edu/infocenter/vitamins/vitaminC/

Something else to consider that I learned in my renal course: 2000 mg per day of Vitamin C supplement increases urine oxalate (a component of a common variety of kidney stone) by 22%. Men who take 1000 mg or more vs. the RDA of 90 mg have up to 40% higher risk of stone formation. So people who are either at risk of forming calcium oxalate stones or have a history of forming them should be instructed to stop vitamin C supplements.

Mind you, everyone’s risk for forming kidney stones is widely different, so just take the above as a cautionary note of something to keep in mind. :-)