Having been used to around two hours of battery life on regular PC notebooks, I bought an extra battery with my iBook back in August. I think I've used it once. Or two times, at the most.
First of all, the sleep mode on Mac takes almost no battery. You can sleep for hours and hours and it might have chipped a percentage or two of the bar. But the really amazing thing is how good the battery management works with varying tasks.
Vision Engineer gives us the precise scoop:
During our tests, the default power saving settings were used. Doing nothing more than typing in TextEdit gave a battery life of 4hours 27mins. Maximum power drain was then tested by keeping the screen on, preventing the hardisk from powering down, and running the distributed.net client [i.e. 100% CPU utilisation]. Using this setting, the iBook kept going for 2hours 47mins.
For students, like me, that use their laptop for notetaking in class, this is incredible. Add in the few regular sleeps during recess, and the iBook can easily go on for an entire day without a refill from an outlet.