I'm seeing eggshell, in the soil, for years, which leads me to believe they are mostly inert.
They would conceivably be broken down, in a highly-acidic soil, or mechanically, as when a compost pile is turned-through. Cacti and other rock-loving plants have bacteria, which will allow them to break down stone, and, in theory, the eggshells.
I find that these processes occur faster, where there is lots of life and input of energy (manual labor.) For instance, I have a mortar and pestle, that is just used for plants.