Like we mentioned above, running shoes can wear out more quickly than you'd think. The impact with the running surface is multiplied by your weight and speed. When you consider this over the many miles you run, well, it's no surprise that you may be replacing them fairly often.
The first sign that it's time to replace them is pain. If you feel any discomfort in your feet, ankles, knees, or hips, you may want to try a new pair of shoes to see if it fixes the problem. If it does, then it was time to replace them.
The second sign is that you feel more tired after a run than you would normally. Your shoes could be slowing you down or they could be making it harder for you to get through each stride.
Another big indication is traction. If you are losing traction on surfaces you had no problem running on before, take a look at the tread on your running shoes. If you notice any of these issues, get a new pair as soon as you can. Wearing worn out running shoes can cause injury, both acute like a sprain as well as chronic, like if your muscles have to work harder or improperly over a longer period of time.