jsTree - returning the value of the node level

I have a jsTree (with checkbox) as follows and wish to return node level value of any checked item:

• Group 1
  • Group 1 A
  • Group 1 B
  • Group 1 C
    • Group 1 A A
• Group 2

The node level for Groups 1 and 2 would be 1 and for Group 1 A, B and C would be 2 and for Group 1 A A would be 3

However, I cannot fathom how to return this value, tried but no joy:

 $('#jstree_demo_div').jstree(true).get_node.length;

Any suggestions welcomed!