Important
YouTube is being constantly updated so this may no longer work at all or as intended.
This code is unlikely to be updated regularly or at all to keep up.
Displays the total length of YouTube playlists on their pages (e.g. https://www.youtube.com/playlist?list=PLeMK2ffi7KQV3EWSzuMI4j-KbGG7JfJKd (Prostrate at the Feet of Giants by Fred Gandt)) and on their sidebar when playing them (e.g. https://www.youtube.com/watch?v=ylRkDv4ofqQ&list=PLeMK2ffi7KQV3EWSzuMI4j-KbGG7JfJKd (Why Can't I Touch Carolyn's Rock Steady Puppet Fingers? by Fred Gandt))
The total time is calculated from the lengths of the videos listed in the HTML which, for long playlists, may require scrolling to load them all before the true total is displayed. In some cases, very long playlists may never be entirely displayed and in those cases, the script will not be able show the correct total time.
YouTube's HTML holds to no common standards (literally hundreds of identical #ids for example) so reading the page as is shifts about is quite fiddly. There almost certainly are better ways to go about this and someone else may have done that; if you can find a nicer script, I recommend you use it instead of this kludgy mess ;)