This extension unobtrusively displays the total duration of YouTube playlists. YouTube Playlist Duration is great for easily viewing the duration of playlists for online courses, music albums, podcasts, and more!
Known issues:
1. Occasionally, the page must be refreshed for the duration counter to update.
2. Try restarting Chrome if the extension doesn’t load.
3. Extension may not work due to pre-installed ad-blockers.
4. Click on duration box if it doesn’t disappear.
Source code:
https://github.com/sridhama/youtube-playlist-duration