Really? I go mad if anything does noise, I can't imagine leaving something turned on for that purpose... I dream of the day when I move to the countryside...
Probably no more than 10 GB or so a month, maybe 20 or 30 at my heaviest usage. I don't download much, nor stream anything. Pretty much forum/article surfing is all I do.
I remember butting up against Comcast's 250 gb limit quite often when I was single (lots of HD Netflix and other types of downloads). Probably around 50 gb per month now.