Most people find about 300-500 - some much less, some much more. A strong indication that they need to be replaced is that you get sudden unexplained little niggly pains with no other explanation.

It'll depend greatly on the shoe. Luck will also come into play a it. Some people who swear by a single type of shoe will sometimes get 400 miles on a pair, replace them with the identical model and then get only 300. My last pair was a pair of flats that yielded less than 300 miles which is, form what I understand, pretty good for flats. The most durable shoes I have ever owned were the NB 755, but that's just my experience.

I get a slight ache in my back when my shoes run out of cushioning. If you don't notice any changes in the feel of your shoes, they're still good.

You're most likely going to get a wide range of responses on this.