First is if I really love the formula, because if I personally LOVE a product, then I am more apt to keep it regardless of its utility to me as a reviewer. If it’s exceptionally bad across the board, then I don’t bother keeping at all because who even needs to hold onto to compare later on?
Otherwise, I look at the brand, whether I feel like it’s popular enough to merit keeping for potential comparisons or being able to refresh my memory on formula later on down the road. If it’s a limited edition product, I am less likely to keep these days. I don’t have room, and I don’t go back into the archives nearly as much now as I did years ago (because my photo quality is more consistent and has improved over time).
For foundations, if it doesn’t match me, I usually declutter because I will not wear it. I try to keep an assortment of popular foundations/concealers on hand in my shade range (or there about), and then I’ll just declutter when they get discontinued or a brand comes out with a new formula that seems to be more well-received. I avoid keeping liquid and cream products, whether highlighter, blush, or bronzer, unless it’s a shade I absolutely love. Then, I’ll keep it in and try to use it occasionally for photos or the like. They just don’t keep as long as powder-based products, so I try to pass them along while they’re still good.