When I got my new jazz in January, salesmen told me when warning light first comes on you have about 8 litres left in the tank, but when it starts flashing fill it urgently, haven't ran it down to 0 miles so don't know what's left at that point, but it would be reasonable to expect them to leave you some margin for human error or distance to petrol stations etc.
Only one way to find out when it runs out of fuel, if anybody does it, let us know....