Vit C is very safe. It is a water soluble vitamin that is not stored or manufactured by humans. What the body doesn't use is eliminated via the kidneys. If I was giving 1000mg daily, I'd divided the dose and give 500mg in the morning meal and 500mg in the evening meal.
Vit C has been advocated by a multitude of people, for a variety of conditions, and a host of reasons. At seven months of age, panosteitis comes to mind. It all depends who you ask and whether you are using it for treatment or prevention.
It will be interesting to me to see some more responses to your question.
I feed booth my dogs Vit C.
BUT, I recommend you feed a kind made primarily from Ascorbate, not Ascorbic Acid, which can cause stomach upset and/or the squirts in dogs if feeding a bit more than they can tolerate. Like Phil said, divide the dose in two.
Here is my take on Vit. C. Dogs do make their own but maybe not enough with all the pollutants out there.
It is said to help with stress, healthy teeth and gums, breaks down protein, strengthens the immune system and helps in wound healing.
I give powdered Calcium Ascorbate at both meals approximately 600 mg total.
Start of with less and work up slowly as Vit.C can cause loose stools.
Hope this helps!!