The leather soles are designed to wear out and be replaced, much like the brakes on a vehicle. Your mileage will vary depending on conditions of use, your weight, and walking style. Give them a break every other day to allow the leather to breathe. Avoid exposure to water and excess moisture. When needed, a cobbler can perform a full resole and heel replacement.
To extend the life of the leather soles, have a cobbler apply a rubber sole saver upon receipt. The uppers of your Chisos boots will last decades if you take care of them. Condition the leather about once every three months. These steps will ensure your boots are ready to hand down to the next generation.