Moss grows about a millimetre or two a year under good conditions, so a wall could be covered in maybe five to ten years if it stays moist and shaded. In a drier spot it could take twenty years or more, and if it’s exposed to too much sun or wind it might never fully carpet the surface. It’s a slow, patient rewrite of the stone, not a quick makeover.

If the water stops trickling, the micro‑climate has shifted—maybe the wall’s humidity is dropping or the airflow changed. Put a hygrometer or a simple humidity log on the wall and watch the data curve. If the moisture drops, the moss will stall, and the stone will keep its old face. That’s when you know something has moved.