I bet we'll never get a consensus on this one. To a great extent it's simply subjective, and there are a whole lotta subjects with a whole lotta different views.
IMO durability refers to how long any imparted characteristic of value remains at an acceptable standard.
With the characteristics and their measure both being in the eye/opinion of the beholder, opinions are simply gonna vary. But there oughta be *some* point at which we can agree- changes occur and are noticeable....slickness can only drop off so far before a panel just isn't "slick" any more; beads change from smaller to larger and from nearly-spherical to more irregular shapes, etc..