Some companies give an estimate of how long it will take you to see an effect. Don’t believe the numbers. You can only get numbers with a large quantity of “subjects” and a double-blind test, which these companies didn’t do.
On the other hand, I can give you anecdotal evidence that I saw. The active acne in my grandson’s chin went down (yes, it was like a hill) overnight with Zit ender.
I had a nasty scratch caused by dry skin followed by scratching (don’t!) and in a couple of days of using nourishing serum with hemp extract, it disappeared. But this is because they were the right products for the problem. If you start using peels and copper salts, don’t complain if your skin looks worse after a couple of weeks. It’s supposed to.
My favorite “miracle” was when our collagen serum lifted my floppy eyelid in a week or so. Not surprisingly, when I don’t bother to use the product, it doesn’t work.
So there you are: we are different, our skins have different needs, some products are better than others for a specific problem so don’t expect hard numbers. If somebody is gibing you hard numbers, they are getting them out of a hat.