I used to use 2GB or less each month..
until I got an LTE phone with Verizon. Then with the 25X to 30X faster speeds I could do much more. I could finally start using the Amazon Cloud drive for my music since it could easily download fast enough for me to listen to the music without any pauses.
So I recently uploaded most of my music, and I used 9GB last month from me listening to it. And September will probably be 14GB to 20GB of use.
I'm still grandfathered on unlimited data. And the only time I would connect to Wi-Fi is when I'm home since that FiOS connection is much faster than a 4G connection. A typical Wi-Fi connection in the wild is extremely slow. So unless I'm on 3G or have no signal, I'll keep using data from the 4G connection.