Depends on the person, the shoe, and what you use it for.
I'm hard on shoes. I rarely make it to 300 miles. I usually need to replace at around 250 miles - last ones made it to 283 but they were pretty done by then. I cannot even wear them to walk a mile in anymore.
Some shoes have more support, and the support lasts longer. Some don't last as long. I even switched to "barefoot" shoes for awhile, but that didn't work for me at all.
So, what happens? Well, things start to hurt. Knees, shins, feet, ankles. Depends really but usually feet and knees. I use my running shoes for only running, when they wear out they can sometimes be worn to work (casual) or walking.
I wear my last pair's running shoes only for weights (they are too thin for walking), the ones before that for walking (different brand).
A couple of decades ago, I participated in the Breast Cancer 3-day walk a few times. Same back then. I'd train 1000 miles a year, and would go through 4-5 pairs of shoes. I'm hard on shoes.