Yes - miles
I never go over 350 miles for any of my shoes...also the companies that I test shoes for never have me run more than 300 miles for a test. Also, IMHO I firmly believe you should have at least 2 pairs (from different mfg) in your training rotation. Currently, I have 4-pairs in my rotation, and will go through 9 (or more) pairs for the year. After 350 miles, I wash them and either use them for lawn duty or mall shoes - or for the most part donate them at the end of the year as they still make good casual shoes - but not for running.