Here's some quick notes I read today about when to replace running shoes.
For typical training-style running shoes, they are good for about 500 miles (in contrast to racing-style running shoes, which may only be good for 200-300 miles).
Also, watch for compression lines in the soles of the shoes. If there are getting to be alot of compression lines, then it's time to replace the shoes.
Another sign that the shoes are wearing out can be the onset of various aches and pains due to the shoes losing their ability to absorb the shock of each stride.
Looking at my training log, it looks like I have about 300 miles on my shoes. I'm running about 20-25 miles per week, so I should expect them to last at least another couple of months. My next race is mid-October, so I also want to make sure I have plenty of time to break in some new shoes before my next race.