dynamically limit depth of wp_list_pages

I guess that this should be an easy one but I haven’t yet find the proper solution.
I want to limit the depth of wp_list_pages so as not to display the pages of the last level. So supposing that parent page A has 3 levels of pages I only want to list first 2 levels. I was looking for a function to count the maximum depth of children for a page but no success.
Till now my code is

$parents = get_post_ancestors($post->ID);
	if (count($parents)>1) {
		$parent = $parents[count($parents)-2];
	} else	{
		$parent = $post->ID;
	}

	$args = array(
            'depth'        => 0,  // this should be set dynamically
            'child_of'     => $parent,
            'sort_column'  => 'menu_order, post_title',
            'post_type'    => 'page',
            'post_status'  => 'publish'
        ); 

		$page_nav = wp_list_pages( $args );

Any help is greatly appreciated