We can walk along the Linked List and count the number of nodes we've seen so far. We'll return the count at the end.

def listLength(head):
    count = 0
    node = head

    # while the node isn't null
    while node:
        count = count + 1 # add 1 to count
        node = node.next # step to next node

    return count

Time Complexity $O(n)$. We have to step through every node in the Linked List. If there are $n$ nodes in the Linked List, we have to do $n$ total steps. This means the total number of operations we have to do is $O(n)$.

Space Complexity $O(1)$. Our algorithm uses 2 variables, called count and node. This means we have to store $O(2)$ variables, which is the same as $O(1)$ variables. Remember, $O(1)$ means the memory doesn't depend on the size of the array. As $n$ gets big, the amount of memory grows like the number 1 does - not at all.