I am on my second pair now, and with wear time totalling probably six days out of every week, five of those through ten hour work days (I literally don't wear any other shoes except the odd evening once a quarter where dress shoes are needed) the tread seems to last around a year a pair. The older pair is still used for 'softing, there's just enough grip left to be useful in the woods.
The newer set seem to have lasted a bit better though, probably got another three months in them. Great boots, really comfy, robust zipper, good safety toe and good grip everywhere but snow and ice. Not e