Anything above 500-1000mg of vitamin C as a single dose is not used up entirely, but rather wasted in large amounts. I seem to remember that 500mg is absorbed entirely, 1000mg about 90%, 2000mg about 75%, 3000mg about 60%, 4000mg about 50% absorbed and so on.
So I take it after meal, and never more then about 1500mg at one time. Otherwise you are urinating some pretty expensive liquid...
Taking 8000mg at one time is about the same as taking 4 x 1000mg during the day.