Strava sends you a nice little message when the mileage on your running shoes reaches the point that I imagine shoe manufacturers want you to go and buy some more. I have always seen a maximum mileage number as a bit arbitrary myself, as it depends on so many different individual variables.
When I ran in road shoes I could tell when the soles had gone ‘flat’ and I needed some more. It varied, but 500 miles was decent for me as a neutral gait runner.
With trail / fell shoes, the uppers never stay in one piece long enough for me to worry about how many miles they have done!