We have GB/mo figures for our customers for every month for the last ~10 years. Is there some simple figure you're looking for? I can tell you off hand that I remember we had accounts doing ~15 GB/mo and now we've got 1500 GB/mo at similar rates per month.
I'm mostly just wondering what others do for this kind of planning - trying to look outside of my own experience, so I don't miss something obvious. That growth in total transfer that you mention is interesting.
I always wonder what the value of trying to predict utilization is anyway, especially since bandwidth is so cheap. But I figure it can't hurt to ask a group of people where I am highly likely to find somebody smarter than I am :-)