Every couple of months or so, I try to go through my boxes of make up and get rid of any make up that I a) don't use anymore and b) have expired.
As a general rule of thumb, I throw out mascara after three months. Mascara tubes are perfect breeding grounds for bacteria that can cause all sorts of optical problems, that can lead up to and include blindness. They're warm, damp environments that are constantly introduced to new elements- such as mold spores in the air, dirt, soap from face washing, and even just general eye germs- during use, after which the brush is returned to the tube where any new bacteria that was picked up can incubate. If you notice a change in the application of your mascara, like thickening or it becoming watery, or if you notice a change in the smell, be on the safe side and toss it out.
With lipstick, foundation, and concealer, you have a bit more time before you should toss it. Like mascara, if you notice a change in the scent of your lipstick (or foundation or concealer) or the texture of it, toss it. The components of lipstick, etc. can break down and separate over time, so you're not going to get an even application and the colour can change on you.
I know, it's a bummer to have to toss out make up- to me it's like having to toss a good friend- but your health is worth much more than a $5 tube of mascara.