To determine the shelf life of a product, you will need to look at the box for said product (or a similar product, ie. if you want to know the life of clone e/s you can look at metamorph, just has to be the same type, not color).
There you will see a little icon that looks like this:
This indicates how long after opening, the product is considered useable. In this case, 24 months for a tube of MAC paint.
Also, check out the FAQ post about cosmetic shelf life, in the FAQ section of the forums.