How far do you use running shoes? 100 miles?

300-500ish depending on shoes, running surfaces, bodyweight. At 135lb I used a pair to 1,000mi. Right now at 182 (powerlifting) it's going to be significantly less. Everyone's different.

Wow. If I replaced my shoes every 100 miles, I'd be broke...

I think there are a lot of variables in how long running shoes last; including shoe, terrain and person variables. I think body weight has a big impact, maybe the biggest. I'm light so my shoes last a long time - I retired my last pair after 1600 km (1000 miles), and my current pair are up to 1200 km. I have had no injuries, even during HM and marathon training. I track shoe mileage with garmin connect, too, as others here have said, but to me, it is more a matter of interest to see how many kms they have done, rather than a prompt for getting new shoes. I usually replace my shoes when they have holes, and the tread is so worn you can feel rocks jabbing into the bottom of your feet. My shoes are Asics Gel Nimbus, and fairly expensive, but I get a lot of wear out of them. Although I think good shoes are important, I believe that often, injury has less to do with shoes, and more to do with poor form and technique, increasing mileage too fast, or running too fast too often.