Comcast got in trouble a few years ago because they began to harass and boot-off what Comcast considered "overly high usage" users. You'd get a warning phone call where they'd tell you "you used too much bandwidth" and then be told not to "use too much bandwidth again or you'd be kicked off the system for a year".
When users asked "What is acceptable bandwidth?" their answer was "Less than what you used" and that was about the only answer you could get. Naturally users would freak out, be afraid to do anything, etc....this began to happen to more and more users, around the time when users began to complain about traffic-shaping that Comcast started doing (throttling p2p transfers, etc).
Ultimately, Comcast were forced to come up with a number for a 'cap'....250gb a month for home users of any speed tier was the number and, frankly, it was fairly generous, considering at the time other cable providers were talking about caps of 10-20gb per month for lower speed accounts or 40gb for "high speed' accounts....but those I think disappeared when people revolted and realized if you even streamed a few movies from netflix, or downloaded a few things (legal or otherwise) you'd hit your cap, and the overage charges were either insanely priced or you'd get booted from the system (supposedly, as I said I dont think this lasted long).
Supposedly today with Comcast if you go over 250gb historically, if its not a lot they wont lecture you...if it is, you run the risk of getting the "phone call"....although now I am told they suggest you move up to the BUSINESS class tiers, which do not (currently) have any cap.
Although...it has been said...if you are doing multiple terabytes in a month, you'll get booted off business class and told to pay for your own enterprise-class line ($$$$) instead.