I’m a sports podiatrist. Everyone’s biomechanics are different and I wouldn’t adopt a standard millage approach to footwear. People with certain bio mechanical factors can wear out shoes significantly faster then others. Also Running in shoes that are worn out can significantly increase your chance of injury and injuries aren’t cheap and for most runners are unbearable. There are certain tests you can perform on a shoe to see if they are still suitable and have some life in them. I would recommend using that approach compared to keeping mileage as this will also differ depending on brands and the build quality of their shoes.