I use my D7000 with the stock EN-EL15 battery inside and the MB-D11 battery grip with a third party EN-EL15a battery which is 4200mAh. I do not remember when I last charged the external battery... maybe late December? There are at least 3000 shots on the external battery, and the internal battery shows over 1200 shots and is still at 63%.
So yeah, I love not having to worry about charging batteries, sometimes for months at a time when I'm not shooting very heavily. :) [Love everything else too!]