When do you retire your shoes?

Do you retire at mileage or tread? I usually aim for 500 miles on my trainers which tends to work well for me. However, I just got my new SB2 and the tread and sole is so much better than my SB1. My SB1 have 575 miles on it and my plan was to use those after raining days and let them get dirty and use my SB2 the other 80% of the time. Is there a tread test yall do?