Calculation updated:
It will also be shipping with 2,050mAh battery as standard, compared to 1,650mAh standard on current Galaxy S II.
Since they claim the new chip draws 20% less power, that means 1,650mAh of the last generation is equal to 1,320mAh.
That means it should last 1.55x as long as current Galaxy S II phones while delivering 2x the processing power.
2,050mAh / (0.8 x 1,650mAh) = ~1.55x longer battery life while delivering 2x the processing power, Best case scenario.
2,050mAh / 1,650mAh = ~1.24x longer battery life while delivering 2x the processing power, Worst case scenario.
This worst case scenario is assuming that Samsung when claiming it draws 20% less power was using the new 2,050mAh battery in it's calculation which could inflate battery performance a bit because some of that "20% less power" Samsung mentioned could be accounted in the larger standard 2,050mAh battery. In that case there would be no need for the "0.8" part in the earlier equation.
My guess is it will fall somewhere between 1.24x to 1.55x longer battery life while delivering 2x the processing power of the Galaxy S II series.
Keep in mind that you can also underclock and undervolt your processor because that "2x the processing power" statement gives one a lot of room to maneuver.
You should also be able to underclock/undervolt your processor to "Galaxy S II like" performance using SetCPU, and your battery life would be 2.48x to 3.1x longer while delivering the same performance as last years Galaxy S II series.
All this calculations are in "theory" of course, so don't quote me on that. Also, keep in mind this new phone has a bigger screen also.