If I have it in airplane mode I get about an hour per 5% of battery life. I've noticed that the battery drains a good bit when sleeping if you don't have it in airplane mode. It periodically wakes up and checks for new content. I keep the touchscreen at very low brightness (10% or less) and don't use it much. Once I'm reading something I generally don't leave it until I'm finished, which can be a couple of weeks. However, if I'm taking a lot of notes that obviously cuts into battery life. If you're using the touchscreen, opening and closing books and keep the antenna on, 10%/hour sounds reasonable.
"dread and fear should not be confused; by dread I'm inspired, by fear I'm amused."