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 lip products, it’s really about a formula that I’d actually want to wear again. I know I very, very rarely reach for a liquid matte or even full-on matte lipstick, so I find I pass those along sooner rather than later. I like a lot of colors, and since the cost is sunk for me, I will keep entire formulas over just a few shades if I really love the formula. It’s very rare that I’ll keep one shade and not its siblings!