It seems more natural to say endless "lengths" of cable rather than endless "length" of cable. Why do we need to make it plural?
A length of cable is an item that might be a five foot bit of cable, or a three foot bit of cable, that is to say, a piece or portion of cable.
If you say lengths of cable, then you're talking about multiple pieces.
So an endless length of cable is a run cable that goes on forever, whereas endless lengths of cable would be a unending number of cables or cable pieces.