The traditional recommendation is 300-500 miles, which, depending on how much you're running, could be one month or six months.

However, in truth, it depends hugely on the durability/construction of the shoe, your weight, the surfaces on which you're running, how injury-prone you are, and the type of running you're doing. For most shoes, the life of the shoe is primarily tied to the material of the sole being permanently compressed by the repetitive impact of running. In other words, your foot will make an indentation in the sole of the shoe, and the shoe will no longer provide an ideal platform of support for your foot. If you pay attention, you can feel this change as it happens.

Some runners will wear a single pair of shoes for 2,000 miles and never get injured. This is definitely a risky way to run, but it happens.