I'd say each boot is good for about 250-300 miles on the rear feet on harsh ground (think sandstone, sharp rubble and asphalt). Boots worn on the front tend not to wear out as quickly so I recommend rotating worn boots from the rear feet to the front feet to get a little extra use out of them (if you can, not all dog foot sizes will allow this).
These boots have all worn through on the middle of the sole, not at the seams. No problems with stitching or other parts breaking lose.
All in all these are very well put together, sturdy boots. They don't fray or fall apart prematurely