If you mean how much we download as in go out and say get openoffice or download something from fileplanet or whatever then on an average month way under 1GB. But if you mean how much is downloaded per month in total use (everything you do online requires up/downloading) then I have no idea. Probably not that much more though.
I easily use up more than 8 gigs a month, and I'm on a supposed 2 gig cap, a lot of that can be downloading various linux distro's to fiddle with, backing up my websites, and just other general usage