Vitamin C last until…you (well, going to the bathroom…peee) it all out, that’s why its extremly important to drink a lot of Vitamin C which can be found in orange juice or even better those vitamin C tablets. It’s not very long, but it’s really good to have Vitamin C in your body, because it will really help get rid of that severe cold and quick and painlessly as it can ;D
Vitamin C is eliminated from your body every day as it is water soluble. The best strategy is to take vitamin C two or three times a day as that is far more effective than taking the total amount once a day. If you only take it once a day there is nothing left for later on towards the end of the day.