In all honesty we as a family are what I would call fairly heavy users, online all the time, kids on youtube/facebook. The wife does all her uni work online using the uni servers and I do a fair bit of browsing and gaming online. On my last check a month or so back we did about 40gb maybe less.
Remember: Amateurs...built the ark.
Professionals...built the Titanic
All spelling and grammatical errors are left in on purpose