Hi Ben,
Depending on what you use your 12GB per month for, separate to any up/down loading of sharing SBS data to networks, it should be more than enough for you to manage appropriately.
I've done a wee bit of searching for you, and from threads available on the Yahoo Groups forum for PlanePlotter directly, users have quoted various different figures in relation to their own research on up/down usage.
Some indicate that that a combined up/down usage per day can average 100mb-200mb and others indicate much less at up/down being 50/50 so 100mb total per day.
Other factors include how zoomed in/out your main map on PP is, which will then determine how many aircraft are displaying at any one time. Their reports are saying that less aircraft to display, then less updates sent/received from the server, hence less up/down usage. It's all relative in the end.
A lot of users in the UK who go out into the field with their ADS-B receivers have said that they all use a 3G dongle option from the 3 (Three) network over there, with a monthly 3GB usage limit. Reports are that it is more than enough for their requirements.
Here's a couple of threads from the PP Yahoo Groups site to support that, though you might need to be registered to view them. It might just be worth your time doing so, so that you can make your own mind up:
http://groups.yahoo.com/group/planep.../message/51534
http://groups.yahoo.com/group/planep.../message/23572
http://groups.yahoo.com/group/planep.../message/18242
http://groups.yahoo.com/group/planeplotter/message/9654
You might also consider purchasing your own monthly dongle to keep that separate from your already limited 12GB, and that way you can manage your own usage much closer and would then be wholly responsible for any up/down usage utilised. In case of a sharing environment at home, at least then no one can blame you for taking a chunk out of the 12GB limit.
This then also gives you the flexibility to run your sharing whenever you like. You might be able to find a comfortable average by running PP for say 8-9 hours per day before you hit your limit, or maybe you'll even get a full 24/7 service out of it. I imagine it would be hard to completely figure out it's exact up/down sharing usage without some live tests on a purely independent system with internet usage set up specifically for that purpose.
I haven't yet researched any related topics on the previous two forums I linked to earlier in this thread.
Let me know what you decide.