In case you still can’t locate it:
As of VLC version 2.1 for Windows and Linux, you can see the total playlist duration next to the playlist menu item on the left side-bar in the playlist view.
(I just spent a few minutes trying to find it!)
VLC version 2.1.2 on Win 8.1
Here is another VLC Player extension to display the total playlist duration: github: bentrevor/vlc-extension.
Once installed, on OS X, it works/looks like this: