Data point:
Sprint Customer on ED1500 plan.
Last 12 months have ranged from 4.8gb to a high of 22.1gb worth of data in a month. Average is 13.2gb.
I travel weekly for work, and am all over the place. Only time I offload to WIFI is when I'm at home Fri-Sun, or if Sprint network is too slow to reliably use facetime. As others have mentioned, and based off of my data usage habits, I can feel like I can safely say that burning through more than 20gb a month on a phone (without tethering) is going to be extremely difficult. I feel like I use the **** out of my phone.