Good morning James,
Thanks for posting, that's a good question. Once you use a token, 500 MB is added to your token data allowance, and yes it rolls over.
If you're in the Anytime Bytes period and you run out of Anytime Bytes, you will start using your token bytes until you run out or until the Bonus Bytes period starts. Say your monthly allowance resets before you use up all 500 MB of your token bytes, you will still have the remaining token bytes for future use.
Hope that clears things up.
Thanks,
Liz