Principles of Playlist Construction: Length of Playlist
I think there are right and wrong answers when it comes to the principles of playlist construction. It's not something I would discuss in public, but if you take any of the major playlist services: Spotify, Apple Music or Tidal, the principles of the same.
The Right Length of the Playlist:
A playlist needs to be long enough so that you do not grow tired of it before you can naturally find new music for that playlist. For example, a playlist of ten songs, where each song is 3 to 4 minutes long, is too short. Amazingly enough, there are plenty of examples of Spotify ITSELF violating these obvious true/false rules. Spotify often provides playlists of pop songs that are under two hours of length. If an average song on that playlist is three and a half minutes, a ninety minute playlist would have something like 25-30 songs. No human being is going to listen to a playlist that short without tiring of it.
I would argue that the optimum length for a playlist is somewhere between 100 and 200 songs depending on the average length of each song. This should produce a playlist of 10 to 15 hours of music, which I would argue is ideal. The idea of a one to two hour playlist is itself rooted in the recording technology of the 20th century- up to and including the downloading of the mp3 album- but does not take into account the changes wrought by streaming music services.
The idea behind having several (1 to 5) 10 to 15 hour playlist is that you have an adequate selection of music to carry you through days, weeks, or even months when you have little time to identify new music to listen to. For example, a student, trying to get through finals, is likely to need a 10-15 hour playlist to help them study and prepare for test taking. Once these playlists have been developed on one device, they can then follow the user into ambient listening situations: in public, driving in cars, using headphones, etc.
Many people have all are part of the working day where they can't listen to music at all and of course there is some period where you are sleeping, so the playlists have to address major periods outside those zones. However, you don't want the playlist to narrowly tied to one specific purpose, you want it to be broad enough to encompass multiple purposes. To give another Spotify derived example, they have playlists like Morning Commute or Evening Commute- both are too narrow Commute playlists are likely to share songs with larger categories encompassing a greater range of music. As I said before, I think the three major playlists are fast, medium and slow tempos- all other topical playlists can be subsumed into one of these three.