Sure, I can guide you on how to calculate the duration of your Spotify playlist. Unfortunately, Spotify doesn't directly show the total duration of a playlist. However, you can use third-party websites or apps to do this. Here's a simple way to do it:

1. Open your Spotify app and go to the playlist you want to calculate the duration for.
2. Click on the three dots (or right-click on the playlist) and choose the 'Share' option.
3. Click on 'Copy Playlist Link'.
4. Now go to a third-party website such as "Playlist Duration" (www.playlist-converter.net/playlist-duration).
5. Paste the copied link into the search bar on the website and click 'Calculate duration'.
6. The website will then calculate and display the total duration of your playlist.

Please note that the accuracy of the duration might depend on the third-party website or app you're using.