- If you use it less than once a month, and it's not for special occasions, or something you truly love, dump it.
- If it's an opened mascara or eyeliner more than six months old, you should throw it out. After about three months, liquid eye products start to gather a lot of bacteria and can irritate you or give you an eye infection. It's for the best.
- Any old makeup sponges should go. After about a week of use, they start to harbor bacteria. Unless you've been sanitizing them every few days, using them can lead to breakouts and contact dermatitis.
- Any opened lipsticks, lip glosses, or lip liners should go if they're more than a year or so old.
- Nail polish separates and starts to break down after about a year, so anything you've got sitting around that's approaching or past that date and isn't on your fingers all the time should go.
- Powders — whether pressed or loose — stay good for about two years. So eye shadows, mineral makeup, etc. have a pretty long shelf life. If you're not crazy about the color, or don't wear it at least once a month, consider getting rid of it, though. The likelihood that you'll want to wear it or get any use out of it before it goes bad is low, and it'll just be taking up space.
- Makeup that's been sitting out in a sunny spot breaks down faster, so if you've got stuff that's been exposed to light or lots of hot days, and it's more than a few months old, you might consider getting rid of it.
- Makeup that was trendy a year ago and isn't something you love for the way it looks on you should go — it won't be back in before the expiration date.
- Liquid foundation lasts anywhere from one to two years depending on the formulation. If you've got older foundation you don't use all the time, throwing it out is probably a good idea.