You are speaking my language. We've got two teenagers at home that basically live off streaming video and game downloads. The general rule is that when they're awake and home, video is streaming.
On a typical school day, we use about 10 GB of bandwidth. Some days, we use much more -- 20 GB in one day isn't unusual. Our high score is about 35GB in one day.
As for how we use that bandwidth, kids do the darndest things.
Sometimes they'll turn on Netflix for background noise, while they download a game from Steam or XBox Live. To pass the time while that game is downloading, they'll start watching YouTube on their iPad.
Sometimes, they'll listen to a YouTube music video while they shower. Teenagers know nothing of quick showers.
Sometimes they'll watch a YouTube video at bedtime, then fall asleep with it playing. Thanks to YouTube's autoplay feature, they automatically stream YouTube all night. Netflix has something similar, but at least you can disable it on an account-wide basis. I haven't yet found a way to do this on YouTube (especially the XBox 360 or iPad apps).
Once in a while, I download a new Linux distro, VM appliance, or OS update, but compared to our streaming video usage, that's probably a rounding error.
In any case, I am not looking forward to the day Comcast rolls out bandwidth caps in our area. Whatever their cap is, we're going to blow it out of the water (unless we can change some habits!)