Perhaps not a better answer but a different one. Most lotions and potions (including sun tan lotion and cosmetics) have a symbol of a tub with an opened lid, by the side of this is a number with an 'M', this indicates the number of months that the product should remain in a usable condition once opened. Unopened, this length of time again.

Unopened cosmetics have a shelf life of approximately 3 years (less for mascara and nail polish - 1 & 2 years respectively). Once opened, you should check for signs that the product has passed its best. These may include colour changes, residue and, in the case of powders, clumping. To extend the shelf life of your products, you should keep them in a cool, dry place (away from direct sunlight). If in doubt - throw it away. It will give you a good excuse to buy something new.