I usually run my shoes for around 1100km usually a bit more. The shoes I took the furthest was a pair of saucony triumph 18 that I took to just over 1600km. But they were past their usefulness at the end.
It really depends on the shoe. For me I start to notice slightly more impact (most in my knees) when the midsole starts to go dead. Not to the degree that it hurts or I get any injuries though. After that the shoes typically still look fine, but kind of lost their advantage as a running shoe.