I bought a 10 year old Bad-Boy 60" that had a transaxle concern.  The original owner maintained 7 acres with it all that time.  Found out that the blades were COMPLETELY worn out, almost to a point on each end, thin and cracked.  The fan blades were broken on one tranny, and guess it over heated.  I serviced 26 hp Briggs, transaxles, greased, & replaced spindle bearings, and put on a new set of blades.  I have 150 hours on it now, and it does a fantastic job.  There is no hour meter on it.  I am curious as to how many hours it could have on it.  I doubt that the blades had ever been changed.  If they can  go 1000 hours, I would guess it is in that neighborhood.  Has anyone seen blades that worn out - and how many hours would it take?  It is certainly not safe, and I have never done that.  I have 600 hours on my cub cadet 1500 tractor, and the blades are fine. It has never been around gravel etc.  That would make a big difference I know.