The life span of the sole is how long it lasts on a given boot before needing replacement. This will be dependent upon any number of factors - amount of use, environment, surfaces walked on, gait of the wearer, phase of the moon and the prevailing wind direction. Replace the sole when it is very thin or has already cracked or has holes. Consider heel replacement at same time. Consider applying Topys if you are concerned about rate of wear and cost.