With my Pismo, I used two batteries and easily got 4 hours in with a little time to spare... wasn't cheap for that extra new battery, either. Then the thing dies two weeks later... :-(
But as far as my 12" Powerbook goes, I found that at first I was getting great time, now it won't last through one whole show.. I think I need a new battery from Apple, best I get is like 2:15 on a full charge, with the LCD just about off.
Otherwise, I love the 12" PB... Now to get that Superdrive upgrade that Scuba Jeremy has me convinced I need..