I'd argue that it's much more than 12. That would be ~20 GB/movie. I don't think Netflix's HD streaming is anywhere near Blu-ray quality. My roommates stream Netflix all day long and our usage has yet to break 200 GB. Not that I'm defending Comcast...
250 GB cap seems huge to me though, unless maybe you have a multi-user household. I have a 50 GB cap but it doesn't seem to be enforced (yet).