The less oxygen your makeup is exposed to, the longer it will last, so always screw those caps on tightly. And keep your cosmetics in cool, dark places - heat and light can cause makeup products' preservatives to break down.
If you use makeup with retinol, the retinol will "expire" and stop being effective in about a month. The makeup itself, though, should still be perfectly usable but you will not be getting the benefits of retinol.
Mascara is the item in your makeup arsenal with the shortest lifespan and the highest susceptibility to bacterial contamination. Experts agree that you should replace it every three months. (And remember basic precautions: Never share mascara, and toss any open tubes if you've had an eye infection.)
As a general rule, creamy products need to be tossed sooner than powders. Your cream blush, foundation, and concealer should last about a year. (Oil-based foundation, however, lasts longer than water-based - up to 18 months.)
Powdered products - facial powder, eye shadow, and powder blush, for example - are among the longest-lasting items in your medicine cabinet, and should last for two to three years.
When sharpened regularly, liners - eye and lip are good for up to three years. Thank goodness for self-renewing beauty products.
Lipstick is another one of the makeup world's great values, and should stay with you (barring any changes in fashion, of course) for a good three years.
These guidelines refer to the most common makeup brands. "Natural" products, however, have fewer preservatives and typically need to be replaced sooner. Another tip I've heard several times is that storing foundation and lipstick in the fridge can extend their shelf lives.
Of course, all of these guidelines are trumped by common sense: If you notice any unusual odors coming from your makeup products, toss them immediately. Changes in color or texture (like clumping or separating) are also a sign that you're due for a trip to the makeup counter.
Finally, don't neglect the tools you're applying all these products with - makeup brushes and sponges can harbor bacteria that lead to acne. Some unconfirmed studies have also suggested bacteria may contribute to rosacea.
Wash your makeup brushes and sponges in mild detergent every week (and replace sponges regularly). Always lay brushes flat to dry - stand them upright, and water can seep into the metal bands that hold bristles together, causing rusting and damage to their handles.
