I usually buy new, every time I want to use some.... ;)

I hardly ever wear make up, less than twice a year for sure.

Moni

There should be a date on the packaging or a safe use life indicated by an open cosmetics container.

Here is a link that might be useful: Cosmetic Shelf Life

I keep mine indefinitely just like spices. The exception is if I haven't worn it and know I won't ever wear it. Then out it goes.

I actually dumped some spices some months back and replaced them with fresher ones.
I think makeup probably has stuff in it to prevent bacterial growth, but it seems to me that after a while especially liquid makeup might go bad?
I'm trying to find a line between having safe and fresh products, and wasting money by replacing something that might still be good.

I use all my make up to the end. I don't use foundation but use cream concealer, eye cream, mascara. Everything else is powder or pencil.

I have never had a problem. Of course they would love for you to purge every six months. Good make up is expensive!

I put some make up on daily. My BB cream and lip glosses are actually good for my skin. Mascara is the only one that should really be replaced every few months. I wish that manufacturers would sell smaller containers of mascara so that it didn't feel wasteful to toss. But the idea of using old mascara creeps me out.

I use powder eye shadows and replace the applicators or wash the brushes frequently. I apply BBs and foundation with a damp sponge which gets washed and set out to dry after each use.

I don't buy a lot of different products; I've known women who have a whole drawer (and countertop) brimming with make up. And what is it with those boxes that hold about 20 or 30 different eye shadow colors?