[personal] how long do you use a product before you say “this isnt working” and stop using it?

Beauty Tips For Body Care
I ask because Im wondering if the reason Iv had little success with my PIH all these years is due to me stopping use of a product if I dont se any changes after a few weeks.. only thing I fee I tested extensively has been bio oil. Ive been using niacinimide for about 3 weeks and not seen a great difference, wondering if I should give it months instead of weeks.
Girls Blog 2015
Submitted by Reacher45