Once you already have the sniffles, there’s no evidence that taking vitamin C will reduce the time it takes to get better, usually 7 to 10 days. In fact, the high doses usually promoted could lead to problems beyond a runny nose and cough, including a higher risk of kidney stone, as well as painful stomach cramps and diarrhea. According to the National Institute of Health, most nonsmoking women need only 75 milligrams and nonsmoking men need 90 milligrams of vitamin C per day, an amount more than covered by eating one large orange or a cup of strawberries (male and female smokers need to get an additional 35 milligrams per day). Any vitamin C in access of those requirements will simply be excreted in your urine.
Source: Consumer Reports on Health March 2015