How many mg's of vitamin c should a guinea pig get weekly?

At the pet store where I work, we put one 1000 mg tab in our pigs' large water bottle each time we refill it. The tab dissolves, and they get a little each time they drink. I would worry more about too little than to much, but the symptoms of overdose are just diarrhea, so as long as your pig has normal stools, you're probably not overdoing it.