Update with Doctor Christophe Desouches, cosmetic surgeon in Marseille.
Do your cosmetic treatments stop working over time? In my opinion no. First of all, one has to address the concept of whether or not the skin can reach a "limit", or rather saturation by using the same routine for several months.
Stable treatment with an anti-aging product clearly gives the best long-term benefit. There may be less acceleration in collagen production after a year than there was after a month, but this is a classic concept in cell biology.
The biggest wins will always be at the start. The cells reach a "constant stability" - where the production of collagen is accelerated from the first applications of your treatment.
If you stop applying your treatment, it will lead to a dramatic drop in collagen production. You can even lose what was previously won.
I advise patients to apply their cosmetic care daily.