As far as I can tell from using Beats Music extensively on my iPhone 6 for the last few months: A TON of listening (I work 12 hour shifts and listen for at least 3-4 hours a day for half the month without wi-fi and then a few songs in the car here and there on my days off) is going to set you back anywhere from 2-3 gigs in the month. Beats uses about 2.6 gigs of data for me in a month.