My ISP, when I asked, said they have a "soft" cap of 60 gigs a month. And unless you're a real video freak, downloading them practically around the clock, or else you're running a server, you'd be hard-pressed to come even close to that.
I was running a little under a gig a month on dialup, and now that I'm on DSL I find I'm typically running 2 to 3 gigs. About the only real change in my habits is that now I can conveniently download MP3s from paid-subscription sites like eMusic, and they limit you to x files per month. Plus, of course, I can now look at larger games and other apps which were too big to even think about on dialup.