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.