My iPad 4's battery lasts more than the 10 hours Apple advertises (generally around 12-13), so I don't see how they wouldn't be able to put a 14-15 hour battery in the new models, considering they will be two generations apart.
The A8 is much more efficient and contrary to the Iphone, don't think they'll boost the resolution substantially.
If they don't change resolution or CPU clockc rate, you'd get 6h longer, thus 18h. If they boost the clock rate (which they'll probably do somewhat seeing as there is more space to dissipate heat). You'd probably still get a few hours more.