How long should you wait before buying new running shoes?

As long as possible. Ideally you want to put off buying new shoes until the old ones are too decrepit to function without a good deal of rope and duct tape. Bonus points if big chunks fall off mid-run.

(P.S. I am starting a running advice column called Brandt’s Really Good Running Advice. I will answer all of your questions about running. My goal is to write lots of very useful information about running and slowly build up an audience until a major running company notices me and offers me a job. Please subscribe.)