There is no exact science in determining the expected lifetime of a shoe. There are many variables that can greatly affect the wear of your shoes. Factors such as weight, activity, frequency of activity, the environment they are used in, and also stored in, all affect the lifetime of your shoes.
Generally speaking, the average lifetime of a shoe is approximately four to six months or 300-500 miles for athletic use. However, your shoes can last longer if you are just using them for casual shoes.