I usually go by feel and mileage. If I've been training for a marathon and have a lot of miles on them, I will start to see if I am getting any excess aches and pains after a few months. Most shoes will start to break down after a couple hundred miles, but this can depend on your weight and type of surface you run on. Running on pavement will break down shoes faster than grass simply because there is more impact and stress on them. Listen to your body and if your joints start to hurt after a couple months, it may be time to upgrade to a new pair of shoes.
If you are trying the minimalist/barefoot running style, you don't have to worry about changing your shoes as regularly since there is much less material to break down over time. Simply replace your minimalist shoes whenever you wear holes in them and you have a hard time keeping them tied to your feet!