Well, I don't have hard and fast numbers... But I charged my Cybook fully upon receipt, and the first book I downloaded was Lawhead's "Taliesen", which is approx. 500 pages. I read the book in a week, and recharged it once during my reading, and the battery was probably ~50% when I recharged it. I probably could have gone the whole book without recharging.
For me, I find that to be highly acceptable battery life.