At the moment the throttle speed is 512kbps, so to be candid, it's pretty slow. You'll still be able to surf the web and stream low definition videos, but you'll definitely notice the slower speeds.
That said, very few of our customers hit 30GB in a month, which is enough data to stream around 60 hours of SD or 15 hours of HD video.
You can check your portal at any time to see how much data you've used this month and when the data resets.