Re: Foundation Make-Up
This is a bit long, but it gives an idea of how long we ought to keep cosmetics for once they have been opened.
Lipstick
Hold onto it for 18–24 months, or until the smell and/or odor changes. "I’m not as concerned about bacteria as I am about the oils going rancid," Wilson adds. Rancidity is expedited by heat, so don't leave your lipstick locked in the car on a hot day (or steamy bathroom, for that matter).
Blush/eyeshadow/pressed powders
These can last up to 12–18 months. Clean your brushes often to avoid contamination and make them last even longer. For the most part, pressed powders won't ever really turn bad because there's no oil or water in them.
Lip gloss
Glosses will last for at least 12 months, closer to 18 if it's an oil-based product and you store it properly. But the nature of the product—pulling the wand out of the tube, letting in oxygen, and then dipping it back in with your saliva on it—ups the chances of it turning. Arnett's insider tip: apply gloss to the back of your (clean!) hand first, and then dab onto the lips to extend its glossy life.
Mascara
Since it's wet, mascara's more prone to bacteria growth, Arnett explains. And since it's your eyes, you should be concerned about keeping it fresh and clean. Many people will keep a tube for six months—most brands out there call that out as the magic number. But some formulas will flake and clump before that; others, if you take good care of them, will last a lot longer.
"I keep mascaras for up to a year but I don’t use the same mascara every day and it’s stored in a dry place," Wilson explains. "If you’re not careful about storage then I would toss in 6-9 months." But always err on the side of caution to protect your peepers.
Eyeliner
Pencils, as long as you sharpen them often, shaving away the oxidized and potentially bacteria-laced tip, can last as long as it takes to use them up. So this is where to stock up. "Since you are so close to the eye, you just don’t want to take a chance with bacteria," Wilson warns.
Foundation/concealer/tinted moisturizers
These products are often made with plant oils, so if you're careful to avoid contamination and store them away from heat, they'll last longer than water-based products. Again, a smell and color check will let you know when these have turned—your nose will know if the oil is rancid. As long as you wash your hands first, and use clean brushes or sponges, these should last around 12–18 months.