as warp says it all has to do with the state of your float.
on my 2010 roadster the light comes on when there's 4 bars left on the gauge, which is always at 40-42 miles left when in town and 60-62 when out on open road.
if i fill up at that point it takes 16 litres exactly, so i figure with a 24 litre tank that means the light comes on when there's a 3rd of a tank left.
the best i've seen after filling up is 240 miles, and this was with the carpenter 240 kit. before that the best i remember was 225 miles, butt then again i didnt have the bike only a few months before i fitted the carpenter bits.