It's 2.16 GB/hour at the highest bitrate Netflix streams HD. That's 115 hours to hit 250 GB. At two hours per movie that's 57 HD movies a month.
Sigh... I meant to upmod, not downmod.
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...