So I rode 430km or so at the weekend using the new Edge 1030 + Garmin's external battery (which is specific to the 1030).
At the end of the ride, the Edge had 100% battery and the external battery showed two bars (of 4). I had the thing switched on the whole 400k event and for the 10k or so rides to and from it from the campsite.
So that is about 26 hours use.
Very happy with that. Far better performance than I'd been led to believe.