According to the manual you should have about 7 litres left when the light comes on... but I wouldn't want to test that theory.
I never let my tank get that empty, though. Probably not so vital in a modern car but years of driving older vehicles where c**p will collect in the bottom of the tank and potentially get sucked up and into the carb (we're going back a while!) or injectors make it a hard habit to break.
I believe that you can damage the cat too if you run out completely.
Apart from that why risk the inconvenience - you'll end up putting in the same amount of fuel overall whether you wait for the light to come on or fill up at, say, a quarter full. What if you had to make an emergency trip and ran out to the car to find you've got practically no petrol/diesel?
Maybe I'm just too cautious?!