the amount of bandwidth you have used up depends on what kind of activities you do on the internet, just like how it applies to cellphones as well.
i've used as little as 50mb of data on a smartphone all the way up past 10gb. this goes from extremely low usage to what is considered rediculously heavy based on the tasks i've had to do. streaming anything makes usage go way up.
as for the home network, we do not have monthly logging of bandwidth since are on unlimited 50/50 but as far as the last 8 hours are concerned usage is from 43mb per hour or less up to 744mb/hr or more depending on task. since one pc is down right now and one gets used intermittantly usage might peak at around 1.4gb/hr if everyone is streaming video. i would say we are at least in the 200-300gb range per month with average usage, a bit more if heavy usage based on pure estimation. using streaming content is what affects this heavily and the quality of streaming content certainly matters.
unless you're capped, i wouldnt worry about it too much.