How many GBs out of 800 (my current package) does one spend to stream a looping MP3 or choise from Apple Music for about 8 hours?

I agree with them on their assessments.

Average file size for a song would be around 4 MB . Average length around 3 min. So 3 min X 20 = 1 hour 4 MB X 20 = 80 MB . 8 hours then would be 640 MB . So around 1/2 a Gig for 8 hrs if it is just a MP3 file.

I pretty much stream Netflix consistently and I have the 600gb and I only ever get to about 300 to 370ish. That's in a month. So I'd probably say youd be under 100gb for 8hrs.