I don't turn on wifi... which has increased my battery efficiency. Plus... most public wifi my phone would connect to isn't great... and I end up turning it off anyway. I average 30 to 40gb, but since NCAA sports starts (especially basketball) my data usage goes up a bit more. Here is my usage the last few months... including today.
3-5 gigs in a normal month. Sometimes I have to thether a lot for work, or my crappy uverse internet will get flaky on me and I'll go closer to 10. My wife and I share 15.
Usually around 10GB, although I hit 250GB on Verizon's unlimited data plan last month. Since you're obviously wondering why so much, I downloaded my entire steam library to my laptop at a hotel.
Since I was upgrading phones I looked at my usage for the past year and was surprised that the biggest month was only about 350MB. I've had the unlimited plan since 2007 and never really thought about data use. But with the $5 increase in the unlimited plan it no longer makes sense for me. I retired a few years ago and don't use the phone away from home as much now. So I switched to the 2gb plan.