If I recall correctly, there used to be a feature of YouTube which showed the total length of all the videos in a playlist, combined. This was very useful as it enabled you to see the length of a game play-through, etc. A Google search suggests this feature has been removed and no immediate solution presents itself, but is there a way to view the total length without having to add it up manually?
You can use "YouTube Playlist Analyzer". It not only shows you the total duration, but also displays a lot of other information about videos of a playlist/channel, graphically in different views.
Note: It may take longer time, if there are too many videos in a playlist.
Example: a screenshot with red marker on the "Total Duration"
You can try YouTube Playlist Duration Chrome extension.
I made an app to do exactly that, hosted on Heroku: YouTube Playlist Length.
Note: it can find the total length of a playlist that has up to 500 videos. If it has more than 500 videos, it shows the total length of the first 500 videos.