Comcast Help & Support - Frequently Asked Questions about Excessive Use:
"What will happen if a customer exceeds 250 GB of data usage in a month?It would be nice to know how much bandwidth we are using.
The vast majority - more than 99% - of Comcast customers will not be impacted by a 250 GB monthly bandwidth or data usage threshold. If a customer exceeds more than 250 GB and is one of the heaviest data users who consume the most data on our high-speed Internet service, he or she may receive a call from Comcast's Customer Security Assurance ('CSA') group to notify them of excessive use. At that time, Comcast will tell the customer exactly how much data per month he or she had used.
If a customer surpasses 250 GB and is one of the top users of the service for a second time within a six-month timeframe, his or her service will be subject to termination for one year. After the one year period expires, the customer may resume service by subscribing to a service plan appropriate to his or her needs."
If Comcast can measure it, how about sending us a report?