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).
I generally do not keep cream/liquid eyeshadows at all, even if I love them, because I don’t have the time to revisit, so it doesn’t make sense to keep something that does expire/dry out that goes around the eyes. I might hold onto a cream eyeshadow from say, Tom Ford, since there aren’t that many released and they’re so freakin’ expensive, but for something like ColourPop Super Shock Shadows, which I LOVE!!!, there are too many released. It’s like once out of 30 looks that I “return” to products in full!