A 9kg gas bottle can get up to 15 hours of use and a 4kg up to 7 hours, this is on the gas being set to the highest settings. In summer or warmer climates, you probably won't have the gas set this high. Needless to say, it's a pretty long time!
For the Americans, a 20 lbs Tank will yield around 12 hours. It's a little less than the longevity of the Australian Spec unit and that's because we're allowed to have a higher gas flow setting for you. This is a good thing because it means you can get heat more water which comes in handy in cold conditions. Again, this is on the 'high' setting.