In the website DoctorYourself.com: http://www.doctoryourself.com/allergies.html
it talks about how a guy took 100,000 mg of vitamin C in a day when he was stung and there were no symptoms, not even swelling. It also talks about a lady taking 20,000 mg of vitamin C a day and her horse and hay allergies were gone. Another person who is allergic to 72 different things takes nearly 40,000 mg a day and their allergies were gone.
How do you get your body to take that much a day? If you do take a megadose how do you know whether it will cause problems for you like calcium oxalate kidney stones and other problems?
Unless you have kidney/renal problems or history of stones, Vitamin C at high doses (up to bowel tolerance) may potentially be theraputic like you put it. I believe they've tested Vitamin C on rats at ~12 grams per Kg and it killed them. 12 grams of C per Kg for a person weighing 150~ lbs (~70Kg) is a little over 800 grams of C that is potentially lethal. But... each individual is different. Most research states that the body only holds 200MG of C at any given moment with good control mechanisms the oral route. There's also the potential for C to block copper absorption, but improve iron absoption.
I can't imagine anyone taking 100GMs of C in a single dose. Maybe it was broken up in 10GM doses every hour, but the source you've posted does not elaborate how the person took the 100GMs.
It more than likely depends on the individual how much C is tolerable. A concrete recommendation is not a valid way to explain that allergies will end or symtoms will not appear at such-and-such dose of Vitamin C. Many people trial-and-error until C doses hit bowel intolerance, then decrease a gram to prevent it.
Its easy, you let your body tell you how much vitamin c it needs. Build up how much you take each day, until you reach bowel tolerance, then back off a little. The main point of this article is that you can take ALOT of C when you are sick- say for example you have had blocked sinus for years, plus you also have a cold, and you are under stress, then your body can take a LOT of vitamin C before it reaches bowel tolerance. If you are healthier then you will reach bowel tolerance at a lot lower dosage. Most will need to stay on the high dose for weeks then drop to maitanence dose. And you take it in small doses throughout the day, but I find its fine to take 5 gram doses at once.