Got mine back in September; when first using it and loading books on, and leaving wiFi on, I needed to charge every 5 days or so (it used to flash up the low battery thing for at least a day and a bit!) I was reading on it for 2-3 hours every day.
Since I've finished sorting out my library, I only add the odd book from my archive or sideloading when I need it - and turn wireless off apart from a 'quick flick on' every few days or so. I haven't had to charge for about 3 weeks, and it's showing half full still
Definitely changes depending on your use and behaviour!