After long writing sessions, the battery's charge loses 8%-10% every thirty minutes (turning battery saver on didn't seem to make a big difference). Consequently, I would estimate extensive note-taking to drain the battery after 5-6 hours. Of course, this would be a poor result for a tablet, let alone an e-reader. However, if you primarily wish to read on the Ares, then expect an estimated 12 hours of use (it is possible to get two full days of use if you turn off the front light and mainly use the built-in e-reading software).
Another issue to consider is that the Ares Note's battery, compared to other e-readers, drains faster in standby mode. The best way to remedy battery drainage in standby mode is to set the Ares to automatically shut down after 120 minutes of not being used.