Just like food, drugs and more
Unlike food, expired makeup products may not readily poison or make you sick but they damage the skin in form of breakouts and reactions that may leave the skin extremely unpleasant (expired products can damage the skin so bad it may stay permanent).
It important while picking out products to check for production dates and in cases where the expiry dates are 'absent', it's best to start counting from the day of first use and this can only be done if you know actual product life cycle i.e how long a product should last for and in this case; beauty products. (Note that even when products seem intact and 'look good' despite past their life cycle isn't an indicator that they can't be tossed into the bin).
Mascara shouldn't be used for more than three months after opening/first use. It's important to discard mascara after three months as they are easy ways to get bad eye infections.
The mascara wand used everytime (pumped sometimes) and returned into the container is an easy way to introduce bacteria into the rest of the product and this isn't something anyone should take lightly. Afte three months of continuous use, thrash the mascara.
2. Eyeshadow and (Powder) Blush
Eyeshadow and powder blush are able to last up to one year, in some cases they might go higher but with cream eyeshadow and blush the life cycle is shorter compared to powder because there's is a possibility they (cream ones) will make breeding grounds for bacteria.
3. Pencils; lip and eyeliner
Lip and eye liners are able to last up to two years as long as tips are kept fresh and clean, they are good for use for years if they last that long that is.
4. Liquid Eyeliner
Liquid liners should be tossed after six months or less. The cue to tossing the liquid eyeliner is usually indicated by a funny smell, when that happens thrash it!
Lipsticks are good for a year preferably they are kept clean and not shared/passed around. Lipsticks should NEVER be shared, it's one of the easiest ways to transfer bacteria/ infection.
6. Liquid Foundation
Foundation can last for between six months to one year including concealer. It's best to keep applicators clean and avoid sharing them to curb transferring bacteria.
7. Pressed/ Loose Powder
Loose and pressed powders can last up to eighteen months to two years as they are less likely to store bacteria. However it's best to limit applicator use to the owner, where applicators are shared there's a risk of infection.
If the powder smells funny before then by all mean thrash it!