Things that would impact your data transfer the most:
* draw distance
* listening to an audio stream / watching a video stream (includes voice, although not quite as bad as an audio stream)
* frequent tp'ing (you get the hit of downloading everything around you each time you tp)
* crashing/frequent relogging with Clear Cache possibly (my inventory takes about 50-60Mb to refetch, in addition to redownloading everything around it)
Having a linkset that frequently sends up object updates can be an invisible bandwidth/data transfer drain as well. It would be best to uncheck Edit / Preferences, General tab, "Show Search Panel in Overlay Bar" so you get the bandwidth indicator back (the right one in the top right corner) and keep a regular eye on it.
(In case you're interested in tracking down what's causing it)
If it stays lit longer than it has a right to use View / Statistics bar, expand Advanced and look under "Network" where you'll see a breakdown on whether you're fetching Object, Textures or Assets. If it's the first, use Advanced / Show Updates and cam around, you'll see red "cones" for major updates, blue for 'minor' updates (and green for deletion).
Anything that's munching up bandwidth by a few hundred kbits will be a dense fog of blue/red (and it will be anywhere within your draw distance, you don't have to be looking at it). A few every now and then, or even a constant stream of a few "cones" is fine.
You have the choice to then either tp away, remove the item (if you own it, or can convince someone to take it down) or if it's relatively far away, lower your draw distance.
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In case you were just looking for a simple answer

. SL uses about 1Gb (give or take a few hundred Mb) for 6-8ish hours of continuous use for me about consistantly for as long as I've been on SL (my ISP is capped so I have to keep an eye on it

). I do tp around, but not generally that much unless I'm shopping.
It's best to monitor your own usage since it's likely to be different from mine

.