The term "milage" in the context of leather does not appear to be a standard or recognized term in the leather industry. However, if you are referring to the durability or longevity of leather, it can be measured through various factors such as tensile strength, tear resistance, and resistance to chemicals and environmental conditions.

For example, high-quality leathers like full-grain leather are known for their durability and can last for generations due to their intact grain and pore structure, which allows them to breathe and adjust to temperature and moisture. Top-grain leather, while smoother and more refined, may not be as durable as full-grain but still offers reliable performance and water resistance. The quality and longevity of leather also depend on factors such as the animal species, age, gender, and the tanning and finishing processes[5>.