I have 2 4.5kg bottles. When the flame looks kinda small on full bore on the stove, then the blardy bottle is running out. It takes a fair while though as we only do cuppa's and a little bit of cooking on occassion. Had our camper just over 2 years and now need to fill the second bottle.
Easiest way to see how full it is? ... pore some hot water over the side ... feel with your hand for coolness ... weight it ... all ways work with some degree of accuracy, but nothing is perfect except for waiting till no gas is coming out ... then the bottle is usually empty.
Kit_e