Vitamin C, and the four weeks nobody waits
Every active is judged in a week and abandoned in two. Skin does not renew on that schedule, and neither does the evidence.
The most common reason an active “does not work” is that it was given nine days to do something that takes six weeks.
The skin's own clock
The epidermis turns over roughly every four weeks in your twenties, and more slowly after that. A product that changes how cells behave cannot show you the result before those cells have reached the surface. Anything visible sooner is hydration, and hydration is real — it is simply not what you paid for.
What the trials actually measure
The published work on vitamin C runs for twelve weeks, not for a weekend. Pigmentation studies typically report their first significant change somewhere between week six and week eight, and the honest ones say so in the abstract.
Introduce one new active at a time. Two at once tells you only that something happened.
How to run the experiment
Photograph your skin in the same light, on the same day of the week, for six weeks. Change nothing else. It is the least romantic advice in skincare and the only kind that produces an answer.