The low fuel light in my 2009 D40 diesel comes on when the fuel level gets down to somewhere between 10-15 litres remaining. I've heard some say it comes on with as low as 5-6 litres left, others I've heard say 20 litres. If you're getting around the typical 12LPHK (give or take a couple) then a "normal" drive will get you to the fuel station without any drama at all.