Back in Sydney, Australia, my provider is TPG, we subscribed to $35 no limit 70 GB quota cap per month. This was in 2008. This cap is divided between 40GB peak and 30GB off-peak. Peak is usage during "peak" hours, or from 7AM (morning) to 1PM (night), and off-peak is during 1PM to 7AM.
If you are an avid downloader (music, movies) and opens youtube casually, I'd say 70GB is decent enough for a home of 2 heavy internet users (downloads, play online games, watch movies, streams, share videos/files, + other activities) + 2 regular/casual internet users (browse, chat, check e-mails, watch YouTube, share pictures, sometimes streams) + 1 occasional internet user (browse, check e-mail).
I spent about 60 MBs per day on non-Youtube browsing on average day, or 120 MB with Youtube, mostly charging to the peak quota. With downloading activities, the consumption will jump to 600 MB figures on the peak and almost 1 GB on off-peak quota (since you left your computer running while you are asleep). The speed is tempting there, you keep opening tabs after tabs without discretion, just because it loads quickly. Keep in mind, that the quota charges both upload and download traffic.