Help me please!!!

I keep the products until I use them up which for some serums it can be half a year. I don't see how a product can be the most effective for the first 2 weeks because with most it takes weeks to see any results - e.g. exfoliants, retinoids, vitamin c. And consistency is key to finding out what works best for your skin and actually being able to reap the benefits.

IMO, I think basically the first two weeks is the period for the product start showing visible effects on your skin. After that, you're used to the effect and think it didn't give you that wow effect anymore although it gives you the same results. Or, your skin is getting used to some ingredients and not giving any visible changes anymore. So, some people don't consistently use the same product over and over, instead, they alternate their products to get the visible results. Or they might be the case, where people start increasing the concentration of the ingredients.

I don't think the products lose their effectiveness, rather they give you a "maximum effect" after which you cannot see noticeable progress anymore, but they still work to maintain that accumulated progress on your skin. I hope that makes sense 😊

even I use my skincare products until I finish them up.. but also like my winter skincare is different than summers, especially moisturisers, and few serums I use them later and they work the same

Make sure to store theme in cold dry place read the instructions some of the products must be stored in cold place.