Are they just dull, or are they chewed to heck?
I had the original blades and the spares I bought from PT until 2014, so 13 years before I bought new ones that I listed above. I still have those today, but they are getting thin.
So three (maybe four, can't remember) sets of blades in 21 years. I'm happy.
When I notice the tips of the grass are getting shredded instead of cut cleanly, I remove the blades and sharpen them on a bench grinder.
One of the great things about the PT mower being on the FEL arms is you can just lift it up and angle it back for easy access. Of course, disconnect the hydraulic hoses, BLOCK THE MOWER DECK UP, and don't put your body under it.
I can reach each mower blade bolt (and the grease zerks above them) very easily without having to put my body or head under that raised deck.