jquery count li elements inside ul -> length?

You have to count the li elements not the ul elements:

if ( $('#menu ul li').length > 1 ) {

If you need every UL element containing at least two LI elements, use the filter function:

$('#menu ul').filter(function(){ return $(this).children("li").length > 1 })

You can also use that in your condition:

if ( $('#menu ul').filter(function(){ return $(this).children("li").length > 1 }).length) {