80% of the day, I am connected to wifi.
When I'm not connected to wifi, I will only use the new iPhone to browse the web casually (surf reddit, check facebook, etc.)
It won't use any more than you do now, it'll just load faster. The only way it will use more is if you now have time to load one or two more pages since it goes faster.
I'm on Verizon Galaxy Nexus LTE with unlimited data and used approximately 6 GB's of data in just a couple weeks. I tether, and stream videos on the iPad3. It's glorious experience!