While in town, home & office/field locations - I'm on mostly ultra-fast fios and/or mirowave links and used to do about 300 MB of mobile data in the Windows Mobile day, nowadays - about 500 or 600 MB of data (that's low for someone on an unlimited but 3G CDMA plan by VZW) which I do minimal to moderate tethering if necessary. I seldom go over 1GB of data unless I'm on the road traveling by car and staying in places with poor WiFi.

Mrs. L has the same identical HTC global smartphone as me, and since she's not a gadget freak, used an average of 30 MB worth of data each month, mostly for reading emails & MMS, with ultra-light browser usage.