You can expect to run approximately 500 miles in a road shoe with a good level of cushioning. After that, the shoe may not protect you adequately from the effects of impact. Shoes with a lower level of cushioning will need replacing more frequently than shoes with a higher level of cushioning. How quickly the cushioning degrades can depend upon your weight and the hardness of the surface on which you are running.
The added stability in shoes for overpronators will also become less effective over time, but should last as long as the cushioning lasts. Racing flats are made to be lightweight and therefore have less cushioning to start with. The Nike Mayfly, for example, weighs only 135 grams and is designed to perform well for 100 kms of fast running (e.g. ten 10k races).
Otherwise, change your shoes when you like the colour of a new model (only joking!).
Not sure if your shoes need changing? Bring them in to show us.