Generally [source? I've heard it] it's recommended to change them every 600km to 1000km. The issue is not only the upper fabric but the state of the soles, that tend to lose grip and shock absorption and that makes you prone to injuries. But it depends on so many factors (weight, where you run, shape of your foot...)
That being said, the feeling you get using them should really tell you if they need to be changed. If you feel like hitting bricks when you run and your knees feel sore, maybe look for another pair.
I'm sure people at the gym are quite happy to see a dedicated person. Nobody gives a fuck about the state of your shoes, aesthetically speaking.
"Boy" ??!? 😤