Steve:
In my exprience, as long as you use it mostly for reading, sync to calibre once a week ad otherwise stay in airplane moode, battery should last for two weeks... and little more.
Keep in mind: the Boyue Likebook Mars is a Quadcore tablet running Android; no matter how light the use, the OS does take it's toll on the battery.
EDITED: you may read in this
other thread for other info on the subject.
Kind regards,
[INCANUS]