I was playing from a playlist I had offline available the entire night. I have downlad via cellular data disabled. My Wi-Fi was dead. The last image in here shows that Spotify had used 1.71Gb of data in 3 hours time. I don't know what was downloaded, since I was playing an offline available playlist and downloading via cellular data is dissabled.
At 3:08 50% of my data had been usedAt 4:07 75% of my data had been usedAt 5:03 all of my available cellular data had been used