Body chemistry, nail length, keystroke pressure, and all sorts of other stuff factor into wear. My ABS keys typically show only moderate wear even after years of use. It certainly depends on the board, though - my old "clear wedge" apple keyboard is as shiny as a mirror while the newer aluminum one shows very little wear in the same time span, and my WASD keys so far seem to land somewhere between the two extremes.
I imagine I could use PBT keys for decades without showing heavy wear.