[Fey]

It depends on a few things, contrary to popular opinion it's rare for makeup to truely go bad.

1) is it dry or liquid makeup?
~liquid makeup can go bad faster
2) Do you store it in a cool, dark location?
~humidity and light can affect shelf life
3) Do you use clean brushes each time you use it?
~reusing brushes can build up bacteria

This being said, I've had makeup for YEARS with no negative affect because I ask myself these questions and try to keep them in mind. I never store them in a humid location, I use disposable/cleanable brushes, and I rarely keep liquid makeup(excluding foundation) longer than a year.

Another good way to judge is to open it and try it. If it's a dry makeup does it still come out smoothly on an applicator? If not, toss. Does it smell funny? If so, toss.