Difficult to tell. But if I leave my NIT tethered to the phone for 3-4 hours it'll have consumed some 700KB-1MB or thereabouts. It runs the following while connected through the phone:

- gmail notifier. Checks every 10 ten minutes. I think this one uses very little bandwidth.
- telepathy (or at least I think that's what runs the chat stuff). It monitors my gtalk etc. friends (online, offline, idle etc.). Not sure how much this uses.
- email program. It checks my work mail over IMAP. This is the old email program btw, not modest. It checks every 15 minutes. I think this one is using most bandwidth. It looks like it uses some 15-30KB just to check if there's anything new. It doesn't use IMAP IDLE or any other efficient protocol. In my opinion it should really be possible to check for new mail with vastly less transfer of data (my inbox is always very small btw).