Well, I've just finished my first book in it, so not conclusive.
It's 3 weeks since the iriver was connected to my computer for file transfer, charging it as well.
Since then, I'm using it occasionally and turning it completely off when not reading. I have read the above mentioned epub (over 1000 page turns), some 200 page turns of pdf comics as a break, and played around with some mp3's and no recharge needed yet.
I'm in the lower third of the battery though.
If you focus on reading and don't play around with accessing menus, notes, comics, music etc. I think that you could take for 2-week holiday reading without recharge.