Are you hitting stones or dirt a lot? That seems very excessive.

No. I have a relatively small yard. My entire lot is 7k sq feet and
subtract off the house and driveway, I'm probably only mowing 4k
square feet. I can cut it in 30-40 minutes with a toro self propelled
walk behind mower. My blades are soft steel and I sharpen them with a file.
I need to keep the blades fairly sharp becuase this mower is only 3.5HP
and I can tell a big difference with sharp blades on a dense lawn.