Cosmetic shelf-life

The shelf life for eye-area cosmetics is more limited than for other products. Because of repeated microbial exposure during use by the consumer and the risk of eye infections, some industry experts recommend replacing mascara 3 months after purchase. If mascara becomes dry, discard it. Do not add water or, even worse, saliva to moisten it, because that will introduce bacteria into the product. If you have an eye infection, consult a physician immediately, stop using all eye-area cosmetics, and discard those you were using when the infection occurred.

Consumers should be aware that expiration dates are simply "rules of thumb," and that a product's safety may expire long before the expiration date if the product has not been properly stored. Cosmetics that have been improperly stored - for example, exposed to high temperatures or sunlight, or opened and examined by consumers prior to final sale - may deteriorate substantially before the expiration date. On the other hand, products stored under ideal conditions may be acceptable long after the expiration date has been reached.

Source: fda.org