Circa 4h ? As stands in the manual.
The Ac charger is a good idea because you can read while you charge your device. When you use USB the device cannot be used = USB connected mode.
I suggest buying a dual cable USB and AC. Sth. like that :
http://photos03.allegroimg.pl/photos...9/73/823597352
+
http://photos03.allegroimg.pl/photos...1/91/802619138
enjoy ;3