This serum really faded my acne scars after I used two-thirds of the bottle within a month! But I didn't control the amount that I used too well and using too much product causes it to get sticky and leave a white residue the next morning. A little definitely goes a long way but I recommend using it day and night because the faster you use it the better. My bottle oxidized after a month but I did not keep mine in the fridge; I imagine it could last up to two months if you did. I don't recommend using after the serum after it has turned yellow because oxidized serum can actually clog pores and cause more blemishes---because of the fickle nature of vitamin c serums and the dropper design (which makes it easier for it to oxidize) I'll have to give it 3 stars.