i have an unlimited plan with verizon.... but i have kept tabs on my data usage just for curiosities sake. i will share some different numbers i've seen...
normal mobile webpage viewing is a constant for me. assume an hour or two daily use with the numbers i list below. no streaming video such as youtube is the norm with exceptions listed.
i've used as little as 500mb in a month. keep in mind that this requires that you dont stream audio or video at all and keep web browsing to a minimum. if you browse the internet for a few hours a day avoiding bandwidth hoggers... its possible to stay at this level.
i've used almost exactly 2gb in months where i was streaming pandora low quality in my car to and from work and every few days at the office.
i have used about 4gb in months where i switched to streaming pandora high quality with the same usage patterns.
i switched from pandora to tuneinradio (128, 192 kbps quality levels) as well as a little youtube and my data skyrocketed to about 6gb-8gb.
last month i have been playing tuneinradio almost exclusively in the car whenever i travel and about 1/3 to 1/2 the time at the office. i used about 11.5gb.
this month i'm already at 8gb and the month isnt over yet.
i think my maximum is somewhere up around 15gb in a month. my normal radio useage, some youtube or streaming video as well as about 3-4gb in downloads and some pc tethering.
Naturally it depends on usage. Admittedly, I don't really use my cellphone too much to download or stream so even though I have an unlimited plan w/ Verizon (no choice in that matter), I typically use less than 1GB per month. This month I am at a paltry 56.5MB so far.
The most data I ever used as probably around 3.5GB, but I had to work hard to use that much. Playing lots of youtube videos one after another, downloading various large apps (and deleting them), streaming music.... it was hard work.