Swappable batteries? If you just mean the internal battery, you can get around 4-4.5 hours if you turn the screen off. Also, it's not a powerbook so you have to turn the machine off to replace the battery.
As for settings, I have my processor set to highest, the hard drives to never spin down, the system to never go to sleep (bad for taping), and the display to go to sleep at 1 minute. This probably isn't the best way to conserve battery power (with the hard drive and processor settings), but I've found it to be the safest settings to tape with...