Do we have any idea of an estimate on the data usage for streaming Apple Music?
256 Kb per second (plus some overhead, technically). So a 3 minute song should be around 6 MB. It caches the songs, though, so if you listen to a song often, it will only be downloaded once. (and then again if the cache decides that it needs more space and deletes it, so it'll be downloaded again.) If you chose to make a song (or album) available for offline listening, it'll be downloaded right then, and it won't be deleted automatically. If you have low bandwidth cap, it helps to download your most listened to songs via wifi.
It's less than Spotify, though.