That's probably what most woman ask themselves. I know alot of woman who has tons and tons of make-up and probably they have some product that is expired since a long time. Especially those limited edition ones like MAC often does, it's more likely never a good idea to go easy on a product that is said "Limited" and make it last years and years. So,
Cream Based- 6 months
Powder Based- 18 months
Mascara- 3 months
Lipstick- 12 months
Applicators- 4 Weeks
Mascara - Mascara should be replaced every three months, no matter how it smells. Never share your mascara with ANYONE! Repeat. NEVER SHARE YOUR MASCARA WITH ANYONE! People have gotten eye infections this way - it's not pretty! Also, never "pump" your mascara - you're just pumping air into the chamber, which dries it out faster.
Eye and Lip Pencils - Don't share these either (actually its a good rule of thumb not to share makeup at all!) Keep up to three years - sharpen often and toss if they become smelly or crumbly.
Lipstick and gloss - Once again, no sharing! Will last up to two years but the "stay-on" formulas dry out quicker.
Powder cosmetics (like blush, eyeshadow) - Up to 18 months, but toss if its cakey, stinky or you can't pick up color.
Foundation - This depends a lot on the product. If it's water-based, toss after a year. Oil based, about a year. If it has SPF in it though, it has a shorter shelf life so no longer than a year. Once again toss if stinky.
Washing your hands before applying.
Not sharing makeup.
Not adding water or saliva to dilute makeup.
Keeping cosmetics in a dry area, away from direct heat or sunlight.
The product's smell has changed.
It has been exposed to prolonged heat.
There is an obvious colour or textural change.
Mould is evident.
[article taken from Steven & Chris]