[Product Question] If left in unopened container, how long should it take Vitamin C serum to oxidize and turn dark yellow?

Beauty Tips For Body Care
I purchased some sample vials of Skinceuticals CE Ferulic off ebay, I've used it before and know that it turns dark yellow if it's old. I'm always very careful to keep my vit C serums refridgerated and to not leave the cap off longer than necessary so they don't go bad.As a test, I decided to place a few drops in a small clear vial and leave it opened to air exposure at room temp. Surprisingly 3 days later, it's still the same champagne color, no darkening in the amount of yellow yet. I guess this stuff is more durable than I thought, that or the ones I received are fake? If you use vitamin c serum, try it for yourself with a few drops and report your results!
Girls Blog 2015
Submitted by caybo