Most cars warn you if you have 5-10 liters left. The easiest way to find out is to find out your tank capacity, refill right away when the warning light comes on, and check how many liters it takes to fill the tank.

My tank is 55 liters, and my warning light comes on when I still have 10 or more liters left.

Heck, I drive the car till there's just one liter left in the tank, sometimes...