Some people do not know how long to keep a product. Some have had an eyeshadow, or liner etc for years. The safest amount of time to keep a product is six months, any longer and your at risk of causing or obtaining an infection. When a product expires, toxic breakdown of compounds can appear which can burn or irritate the skin. Any makeup that you have touched directly should be tossed sooner than anything dispensed by a pump or applied with a brush.
If your foundation is in a pot and you use your fingertips to gather the product, you'll need to get rid of it sooner than a foundation in a bottle with a small opening. Bacteria from your fingers can contaminate your jar.
Pressed powders have a longer life than creams of liquid products. So use your discretion- If you have had that blush since 98, and that lipstick since you were 16 throw it away obviously. But if you have had it a year or two and it seems ok you can hang on to it alittle longer.
Lipgloss can go rancid and you can tell immediately. It will smell and taste awful. Once it does-chuck it. Nothing that smells awful should go on your lips. DUH
Brushes can last for years and years but if you have smashed them beyond recognition or they are broken and frayed, treat yourself to some new ones! Brushes can only take so much abuse and neglect but when treated right you can keep them a very long time. Make sure to wash them regularly.