Ok I will be the first to fly the BS flag. For one 600 hours on 4 machines, really thats mowing (using the machine) 8 hours a day for almost 4 months. Now all of us know that out of a 10 hour day we may run a mower 6-7 hours on a tight route.
second point, a mower hitting big stuff would require the blades to be sharpened in order to leave a decent cut. I don't know your business but if I didn't change blades every week, people would start calling me and asking why the lawn is brown from frayed ends!!!
What ever I do I must do it right!