300 - 400 I would agree is a good number.
It depends on many factors:
1. Quality of the shoe. Don't cheap out here - a good shoe will help prevent knee and ankle injury.
2. You - How you run (i.e. how you land, how you push off, do you pronate, your stride (a 6 foot male takes significantly less steps in 300 miles than a 5'2" female although he is likely much heavier), how much you weigh
3. Where you run - 400 miles in airconditioning on a treadmill which gives and cushions your landing will not wear a shoe nearly as much as 400 miles on concrete in summer heat, winter cold, rain, etc...
4. How fast you run/walk.
5. Your tendency for injury.
If your knees hurt, your ankles hurt, your feet hurt, or you start developing tendonitis - check the mileage on your shoes and the quality of your shoes. Go to a running store (staffed by runners) that will put you on a treadmill and watch you run. They can recommend the correct shoe for your running style.
If your shoe is much easier to twist in your hand than a new pair - probably time to replace.
Walkers can typically get more distance from their shoes since walking puts less stress on the shoe.
Even my 5'4" 100 pound wife starts evaluating for new shoes around 300 miles. She may keep the older shoe for short runs on a treadmill (for a while) and new shoes for longer runs (whether on street or treadmill).