Here's the deal.....
The life of a shoe can range from 3 months to a year. Most typically I tell someone a shoe's life is 6-8 months. There are a lot of factors that go into the life of a shoe. The runner's weight, how often they run, do they use the shoes as everyday shoes, the types of surfaces they run on, the kind of shoe, and many more factors determine the life of a shoe.
Here is something else to consider. Shoes are made with oil, rubber and foam. These are not indestructible resources. Look at the tires on your car. They don't last the life of your car, they need to be replaced as you wear them down.
I work with people who have had the same pair of shoes for years, and tell me they hate running because of the pain they deal with. A new pair of shoes can go a long way to getting rid of the pain.
The elephant in the room is the cost of shoes. I understand that paying $100-$150 twice a year can be costly. Yup...I get it. I also get that if you suffer from physical ailments the health care costs will be substantially more. Physical therapy, orthodic insoles, doctor visits all cost more than the pair of shoes.
Now I know you are thinking, "Charlie, you work for a running store, so you get shoes pretty cheap, but what about me?" I do, no doubt about it. My recommendation is if you have a pricier shoe, put a little money aside for shoes. I do that, even if I am paying less than the average runner. It's my running account.
The fact is running is still one of the cheapest sports out there. The equipment is minimal, and the benefits to your life are so amazing. It's all an investment.
Charlie