I don't know if it's changed with the G4 models, but on the G3s the 12" models said around 5 hours and the 14" models said 6 hours.
Mine lasts about 3.5 to 4 hours under moderate usage. To me that's running Photoshop, InDesign, and 2 or so other apps. If all I do is run Safari and Mail, then I might see 5 hours on a charge.
Check your energy saver preferences, you can trim alot there.