length - a non-negative integer, or nil.

 (list-length '(a b c d))  4
 (list-length '(a (b c) d))  3
 (list-length '())  0
 (list-length nil)  0
 (defun circular-list (&rest elements)
   (let ((cycle (copy-list elements))) 
     (nconc cycle cycle)))
 (list-length (circular-list 'a 'b))   NIL
 (list-length (circular-list 'a))   NIL
 (list-length (circular-list))  0

 (defun list-length (x)  
   (do ((n 0 (+ n 2))           ;Counter.
        (fast x (cddr fast))    ;Fast pointer: leaps by 2.
        (slow x (cdr slow)))    ;Slow pointer: leaps by 1.
       (nil)
     ;; If fast pointer hits the end, return the count.
     (when (endp fast) (return n))
     (when (endp (cdr fast)) (return (+ n 1)))
     ;; If fast pointer eventually equals slow pointer,
     ;;  then we must be stuck in a circular list.
     ;; (A deeper property is the converse: if we are
     ;;  stuck in a circular list, then eventually the
     ;;  fast pointer will equal the slow pointer.
     ;;  That fact justifies this implementation.)
     (when (and (eq fast slow) (> n 0)) (return nil))))