Pandora is reported to stream at 64 Kbps. Minimum 6GB, but that gives no room for any other data use, so I would go with 12. Other music services stream at higher rates, so 6 would not be enough in those cases.
If in your shoes, I would consider a different provider. There are good options out there for low cost unlimited data & even free music streaming if one of those providers have good coverage in your area.
I know how concerning this is for you as you only want to avoid overages. We have a great data calculator page that will give you a better understanding on how much data is being used for streaming music, watching movies, browsing the internet, etc. Please visit us at www.vzw.com/datacalculator
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