my fuel light has only ever come on once. and that was only because i wanted it to, because i was asking myself the same question as yourself, David.
My fuel gauge was actually sitting on empty, and after driving around 100km whilst sitting on empty, the light decides to come on....
So, quite a similar situation to what Baconer has explained.
every time my fuel gauge would hit the empty mark, i would go and fill it; and i would only be able to fill around 40 litres!