most beauty products you buy will have a symbol like the one pictured below stating how long the product is good for, this symbol is usually on the back of the product near the bottom. 12 M= 12 months, so the product should be thrown away 12 months after opening if it's sealed, or 12 months after purchase if it's not sealed. some hair or skin products may not have this symbol anywhere on it, if that's the case then product is generally good for 2 years. watch out for change in color, smell or consistency and if any occur then it's probably time to toss it. expired products can harbor bacteria or mold, go rancid or just not work like they were originally intended, so next time you're doing product "inventory" as i like to call it look for these clues to expired product and toss what's gone bad!
that covers hair and skin products, but what about makeup? in most cases this stuff isn't cheap and it's hard to throw away an eyeshadow just because of the 2 year rule i mentioned earlier, good news you don't have to! there are some exceptions to the 2 year rule, most powder, eyeshadow and blush/bronzer are anhydrous (they don't contain water) and can be kept for years and assuming you don't wet the tip of an eyeliner or lip liner pencil and sharpen before every use they can also last for years. mascara should be tossed every three months no matter what, the wand and tube is a known breeding ground for bacteria. most foundations and liquid concealers can last a year as long as you don't apply them directly to your fingers or face, try a sponge or foundation brush (but remember to wash those often too).