Mech Greenhand posted a battery time of 90 hours with the RGB lighting disabled, and about 6 hours with it turned on. But that was the old BT 3.0 model. He believes that with the new low energy BT 4.0 in the newer models, battery life could be as much as 10x longer, which I think we can agree is a stretch.
However, even 300-350 hours (approx 2 weeks, as fruitalgorithm estimates) is pretty good, and probably due in large part to the fact that the keyboard drops into a low-power state sleep mode after a certain amount of inactivity time (the amount of time I don't know and have never seen documented anywhere). I have found it useful to map a key to the Wake function to make it easier to instantly wake the keyboard up. I don't believe there is a default key mapping to this function, and without it, I have found that pressing a key will initiate some sort of waking process, but it takes a long time to actually wake up and reconnect to my iPad.