It is represented by this jar sign:
which says the number of months the product is valid since the day you opened it.
Let's quickly go through series of make up products, talk about their usual shelf life and how you know these are no good anymore.
Foundations. The validity of this particular product depends on its texture and packaging. The stick, cream and liquid foundations resist from 9 to12 months. The powder foundations are good up to 3 years.
When these products expire, the ingredients start to separate and the germs are easily gathered.
So, be careful with your expired foundations, as these can easily irritate your skin or bring some unexpected break outs or blemishes to your skin.
Mascaras. On most mascaras, the jar sign says 6M, however I would say keep using it for maximum 4 months. Your eyes are the most sensible part of your face and can be irritated quite easily. Just imagine those germs on the mascara wand travelling back and forth, in and out of the tube.
Liners. When you feel like your eye or lip liner doesn't have the same texture or colour, it is time to replace it. The validity term is from six to 12 months.
Eyeshadows. These work the same as foundations, depending on the texture. Cream eyeshadows tend to last for a year,but these usually dry out before that. Powder eyeshadows are valid for around three years.
Lipsticks and lipglosses. Lipsticks last for 2 years and lipglosses around 12 months. If you notice a change of smell, texture or even taste, please throw them directly into the bin!
Fragrances. 3 years is the term for perfumes. When the liquids expire, these usually change their colour and the smell becomes different when sprayed in the air(it doesn't change sprayed on the skin).
An expired perfume may cause skin irritation.