Doses of vitamin C up to 250mg per day are pretty safe. Higher doses up to about 1000mg per day are probably safe if you drink sufficient amounts of water.
Linus Pauling advocated megadoses of vitamin C - he reputedly took up to 90g per day for himself. Whilst the recommended daily allowance is a small fraction of this, the fact that he lived to his 90s shows that vitamin C is not as toxic as you might imagine. Vitamin C is water soluble anyhow and does not build up in the body as long as you are well hydrated.
One known reaction to taking too much vitamin C is an increased risk of the formation of urinary tract stones.