When to Replace Exercise Shoes

I've been using my cycling shoes since October, and I'm trying to figure out how to tell when I need to replace them.

http://www.beachbody.com/beachbodyblog/fitness/how-to-know-when-to-replace-your-workout-shoes
They suggest replacing your shoes every 6 months or 500-600 miles, whichever comes first. They also suggest how to inspect your shoe for signs of wear.
http://www.aapsm.org/replace_shoes.html
Replace every 300-500 miles or every 45-60 hours.