Any boot that is made with a synthetic foam insole will have a life in miles not really time. Even if the upper is still in good condition the foam of the sole compresses and eventually fails. But it sounds like your boot is just overall failing based on your description. Running shoes are 300-500 miles. Hiking boots are 1000. How quickly they fail will depending on how you are using them. The only way to really get longer out of a boot is to go with a traditional construction that has no synthetics in the sole. So a full leather sole will last longer but it will be very different then a traditional work boot as you don't have all that cushion. So essentially with a modern boot you get an immediate comfort which sacrifices longer life. Or you go with something that is traditionally constructed, but you will have to get used to that. Most people's feet are used to all that cushion and support. SO trying to go directly to a pair of them from a more modern shoe construction will be difficult.