As far as I know there's no bandwidth limit although I have heard of Comcast dropping contracts of people who are doing excessive downloading. This would be people who are illegally burning movies for resale and the like.
Man that would stink to get your service dropped for doing what your online for

I suppose what I was getting at was, there is always a file in your modem that allows only a certain amount of bandwidth. (bootfile ) Not really how much you download, but at what speed your modem is programed to run at.
Like my service is w/ roadrunner, they have a 15 Mb down limit, and a 768 Kb upload limit.
In meaning this is what speed I can download , or upload at, and no more. Not the quantity.