On average about 150-300GB/Month. If I look back on my usage page at Giganews, the yearly total drops my jaw.
Then again, all of it is burned off (eventually) to dvd's. I have two large cabinets filled with binders than hold about 150 DVD's each. To store and process I have some 4+TB of disc space, adding to that (with the price of 1.5TB drives...) at least one drive a month now. Going forward on storage (exp. now with all the x.264/AVCHD stuff) is a major headache. Do I add more raw space, go with Raidx off a machine motherboard, go with a network array, or ...??
I've got a spreadsheet where I massage the costs, but for now I'm sticking with adding simple (1.5TB) drives since I have the sata ports in abundance. But at some point (by the end of this year?) I've got to make a major move one way or the other.