Hey there Reneux!
When it comes to downloaded music, they will never use bandwith! Not even online!
So no matter how long the music is playing, the only bandwith Spotify will consume in those cases are music/album/playlist thumbnails (if they are not downloaded) and maybe some basic functionality if you are searching for something. Don't worry! It's minimum, almost nothing! 🙂
Is there anything else we can assist you?
Cheers!
