The interval depends on grit, not on appearance
Carpet does not wear out from looking dirty. It wears out because grit carried in on shoes settles to the base of the pile and is then ground against the fibre by every footstep. That abrasion cuts the fibre, and cut fibre reflects light differently, which is the dull grey path you eventually see in a traffic lane. By the time the lane is visibly grey the damage is mechanical and permanent. Cleaning on a schedule removes the grit before it has done that work, which is why the interval is about protection rather than appearance.