Well streaming a 1080p Super HD movie on Netflix is around 4GB+. That's for 90 minutes of film.
I watch maybe 2 or 3 episodes of Dexter a night with another episode in the morning before work. That's just me using around 12GB of bandwidth a day. My mom uses it as well as my girlfriend and my sister.
So that means I alone use 360GB of bandwidth just for Netflix. There's still Spotify and online gaming. Game updates. Buying digital games.
Using 700GB+ doesn't sound too unreasonable if you aren't torrenting the entire internet.
I burned through roughly 100GB each month on a 4mbps ADSL line in our previous place. We just moved, so I'm waiting for my 10mbps fibre line to be installed. I believe the 200GB data I get with the package should suffice...