My NCs and NT seem to average about 10% battery consumption per hour in use. With power off, they're good for 4-5 days of standby with light usage.
My NSTs are easily good for a couple of weeks of light reading.
Overall, battery life on my NOOK devices has been good, provided I'm careful with what I do with them. If I work to conserve battery, they do well.
My NST, reading about 15-20 hours per week will usually last me around 4-6 weeks with wifi off.
My wife's NT will last about 2 days with on.