According to http://www.apple.com/ibook/specs.html:
iBook with 12.1-inch display: 46-watt-hour lithium-ion battery provides up to 5 hours of battery life on a single charge (1)
(1) Battery life depends on configuration and use.
So I'd guess you'd get around four hours of normal usage. I don't know, I have one of the last coloured generation of iBooks, whose battery is supposed to run for 6 hours on a single charge. I recall playing mp3s on it through a car stereo most of the way from Auckland to Taumaranui when it was new, not that that would mean much to you. It's a few years old now, and lasts perhaps an hour - I haven't run it on battery for a while.
When my power adaptor broke recently, the iBook slept for over a week after 'fainting' from lack of power, so if you only want the pulsating sleep light and staggering uptimes, you could probably have it going for months on one charge. ;-)
