An incredible amount of research exists that clearly demonstrates that excessive Vitamin C will be flushed from the system. It also makes clear that the maximum benefit comes when you split the doses at even intervals throughout the day - otherwise you just give yourself a burst, which your body will almost always be unable to use in its entirety. Linus Pauling came up with the figure of about 50 mg/kg of body mass. This figure is based on studies done of other animals that produce their own vitamin c from glucose. Humans, many other primates, the guinea pig, and some fruit eating bats are the only mammals that don't synthesize their own vitamin c; this is because, as fruit eating animals (or animals that evolved from such creatures), our diet provided enough vitamin c, and so it would be wasteful to use up perfectly good energy (glucose) to make this vitamin. So our (much smaller) ancestors lost the ability to make Vitamin C, and although we can certainly get enough from our diets to avoid scurvy, it is a very long cry from getting optimal doses. So if you're about 150 lbs, you're roughly 68 kg, and your daily dose should be about 3400 mg of Vitamin C. And that's if you're healthy and not under stress. A good measure of how much Vitamin C you should ingest, which accounts for individual variability, is the "bowel tolerance" test. Basically, you keep, daily, increasing your dosage until you begin to suffer gastrointestinal distress (you fart and have diarrhea and other pleasant stuff), and then you dial it down just a bit to before you had these "problems". I have been ingesting just over 5000 mg of Vitamin C (on healthy, low-stress days) based on my body mass for over 5 years, and I am the picture of health. Whenever I accidentally bruise myself, have a stressful day, miss sleep, and/or suffer from the occasional infection (I'm prone to ear infections, partially because I can't avoid being a bit to aggressive with the Q-tips...), I dial up the dose, sometimes as much as 1000 mg per hour, but never past bowel tolerance (which you notice), and I find I recover faster and have weaker symptomatic expression. Incidentally, there has been some research that is very promising wherein patients with advanced HIV were able to recover some immune function by ingesting up to 100 GRAMS (100,000 mg) of Vitamin C in split doses daily. As an oversimplifying rule of thumb, the bigger the mess you're in physically, the more you take over your 50 mg/kg base dose. There is no chemical mechanism that could ever allow Vitamin C to thicken arteries. It prevents oxidation of cholesterol, which then forms arterial plaques.

I take that much or more when I am fighting off a UTI or other kinds of infections, it's perfectly fine. The previous commenter made some very good points, especially about the bowel tolerance.