I'm not on an iPhone (or even on a smartphone - so they won't threaten to throttle me), but I'm currently at 13.9GB with 10 days still remaining. Last month I used a tad over 50.9GB and 10.4GB the month before that.
I realized that I tend not to go over the 2GB threshold. The most I've been to, I believe, is just under 3GB and I seem to average around 1-1.5GB. I should be ashamed for not utilizing the full potential of my unlimited plan...