The 15GB is going to be variable based on the link speed available. If full 5G, that can be erased with 15 speed tests in a few minutes.
From there, it's 128kbps * 3600 (to hours) * 24 (to days) * 30 (to month) = 331,776,000 kilobits -> 41.472GB + the original 15GB -> 56.472GB is the limit each month for "unlimited", roughly. A hard limited number.