Yeah, that's the question!
Humans are not able to make their own vitamin C, but most other mammals can. We can use the amount they manufacture for themselves as a guide for what we should be eating.
Here are some data points:
(1) 70 kilograms (140 lb) of house flies manufacture 10 grams of vitamin C per day. In general, animals manufacture about 10 grams per day according to Linus Pauling. (see here)
(2) An adult goat makes approximately 10grams of vitamin C per day (see here)
(3) Linus Pauling took 18 grams of vitamin C a day, and he lived to be 93. At least that proves that vitamin C is not detremental to your health.
Each of these must have a compelling reason for making this amount of Vitamin C or they would not do so. Vitamin C is essentitally made from sugar molecules and every gram of vitamin C costs at least a gram of sugar.
