How much petrol is left in a 1.2 fiesta when the petrol lights comes on?

How much petrol is left in a 1.2 fiesta when the petrol lights comes on?
When the petrol light comes on in most cars there is enough left in the tank to get to a petrol station, which is around ten or fifteen miles worth. So lets say a fiesta does 40 mpg, that would mean 1/4 of a gallon or around a litre of petrol.