"When the rope is first put into service the outer filaments of the rope will quickly fuzz up (fig. 2). This is the result of these filaments breaking and this roughened surface actually forms a protective cushion and shield for the fibers underneath. This condition should stabilize, not progress.
If the surface roughness increases (fig. 3), excessive abrasion is taking place and strength is being lost. As a general rule for single braid ropes, when there is 25% or more wear from abrasion the rope should be retired from service. In other words, if 25% or more of the fiber is broken or worn away the rope should be removed from service. With 3-strand ropes, 10% or more wear is accepted as the retirement point"
-- excerpted from Samson Rope's Technical Bulletin on Rope Inspection and Retirement (a pdf file)