When creating a YouTube playlist, there are a few length considerations to keep in mind:

1. Number of Videos: There's no strict limit to the number of videos you can include in a YouTube playlist. You can add as many as you want, but for user experience, you might want to limit it to a manageable number. Playlists with hundreds of videos can be overwhelming to viewers.

2. Video Length: The length of the videos in your playlist can vary. You can include both short and long videos. However, consider your audience's attention span and the time they likely have available to watch. If your playlist is for educational purposes, shorter videos (around 10-20 minutes) might be more effective. For entertainment, longer videos might be acceptable.

3. Total Playlist Duration: There's no set rule for this, but again, consider your audience. If your playlist runs for several hours, viewers might not have the time to watch it all in one sitting. A good practice could be to create different playlists for different themes or topics, keeping each one to a reasonable length.

4. Arrangement: The arrangement of videos in your playlist can also impact the viewing experience. You might want to start with shorter, more engaging videos to hook your viewers, and then move on to longer or more in-depth videos.

Remember, the key is to create a playlist that provides value to your viewers and keeps them engaged.