I do the same thing as Clare. No timing...my body tells me. And once I get new shoes my feet and legs say "ahhhhhhhh".The time for replacement is dictated by my body's response to the current shoes. If I am starting to feel aches and pains, in the achilles and knees primarily, then I know it's time for new shoes. I don't time it, I just go by feel. I am always right. And as the other poster said, the new pair is used exclusively for indoor workouts, the old pair becomes the "running around in" pair.
Clare