I once saw a graph that shows the degradation of midsole material over time. After the first 100 miles you lose I think 50% of the shock absorption and then it more or less levels off to around 30% after 300-500 miles. I never count miles on my shoes. Just run until you can feel the stones under your feet or you feel your feet becoming more sore after a run. Also, be careful of lateral wear of the shoe. If the lateral side of the shoe (look at the heel as well) becomes really worn you will basically be landing on your foot at a more pronounced angle, requiring your foot to recover to the midline more (takes more time for your foot to roll toward the midline), reducing speed, increasing the stress on your foot, which could lead to injury. When I see the lateral side becoming really worn then I know it's time for new shoes.
Think of it this way...in new shoes you're still landing on the outside edge but with old shoes the edge is beveled so the landing is at a more pronounced angle. Because this angle is less in new shoes you should be able to run FASTER in new shoes because the foot will take less time to pronate over to the midline.
Alan