ajw:

Slingshot: I would err on the side of caution with those above, as I can attest to how often data can disappear to un-suspected sources. 3-5GB is a good starting point.

To give you an idea on the most common consumer, youtube, 10 minutes of 360p video consumes roughly 100-150mb.
So if you are sparing and do an average of 10mins or so a day of this thats 3000-4500mb per month on that alone.

Extra data per month usually costs more than a base plan with most providers so i'd recommend a higher base plan for the first month (assuming you can change plans later) then scale down based on actual habit, rather than facing extra data charges unexpectedly.

Any definite date yet when you will be launching voice and data plans on the XT network.