There doesn't appear to be any firm number on the absorption rate, there's various studies that point to varied numbers (at least from what I could find). Most of the information I read guides people to a maximum of 2,000mg.
There doesn't appear to be any proven harm by high levels, and any C that isn't absorbed is passed through the urine. The only side effects that may happen by high levels are like diahrrea and whatnot. Of course, I'm sure there are 100 different angles on this.
So far, I've not had any ill effects of my supplements. I may well run into issues as I continue to add additional supplements, but I'm willing to do so to find a way to fight this fatigue.
Some other links I read were:
10/24/2006 Status :
Awaiting saliva test results.
Reading book on adrenal fatigue.
Taking the following (in addition to multivitamin):
6000mg/Vitamin C 800mg/Vitamin E
1000mg/Vitamin B5 100mg/vitamin B6