As long as you're not sharing your makeup with other people, there is a pretty predictable shelf life to your makeup. You can use each type of makeup from between three months to several years, depending on how you use it.
Mascaras and liquid eyeliners can be used and kept for up to three months. They're liquid, and they're bacteria-ridden, so it's best not to keep these around for too long.
Lip glosses, eyelash curler refills, and concealers (the cakey kind) have a shelf life of six months.
Lipsticks, liquid foundations (as long as you're not touching your fingers to the bottle to apply), and nail polishes last up to one year. For lipsticks, it's best to clean it off with a tissue between uses so there will be less bacterial growth.
Blushes, powders, eye shadows, and bronzers can be kept for up to two years. These are the drier makeups, so they will have a longer shelf life. If you are concerned about the cost of makeup, then I would suggest that you choose the drier makeups, such as powder foundations and powder blushes because these will last longer. In other words, avoid anything liquid or creamy, like the 3-in-1's.
Your brushes can also be kept for up to two years, but provided you clean them after each use. I would suggest cleaning them with the MAC Brush Cleanser between each use, and then wash them once a week with Johnson's Baby Shampoo.
You can keep falsies around for about up to ten uses. Pencils, such as eyeliner pencils or lip pencils can be kept for several years, provided you sharpen them between each use.
This is very important because if you are not careful you may cause breakouts on your skin, or even worse, have a bad reaction to the bacteria in your makeup. It's better to be safe than sorry. It may seem expensive to throw away makeup like this, but if you are concerned about the cost, try not to use the makeup unless you really need to. For example, I try not to use my new mascaras until I've completely run out of my old one.
If you are not sure about the age of your makeup, try to look for indications that it's on its last leg. The mascara may be dried out, or your liquid foundation may be separating, or just try smelling your makeup. There will be tell-tale signs that you need to throw them out.
And one last thing: please, please, please do not share your makeup with anyone else! This is so bad for you, I can't even begin to explain! Please, just don't go there. I hope this helps, and let me know if you have any questions or comments. Thanks for visiting!