I am the first to admit that I keep make up much longer than I should.  I do not like to waste things and want to get the most out of them.  Rouge Magazine (you can get it free on the freebies page) had an article about make up and how long it lasts.  Here is what they recommend:

Mascara - 3 months
Liquid Liner - 3 months
Skin care products - 6 months
Sunscreen - 6 months
Liquid Foundation - 6 months
Cream Based Cosmetics - 6 months
Hair Products (except hairspray) - Every year - I guess I better start using all those bottles of shampoo I have
Lipstick - 2 years
Nail Polish - 2 years
Powder Based Cosmetics - 2 years

I hope you have found this as helpful as I did!