The fuel gauge in the car starts to light up: How many kilometers can I still manage “in reserve”?. As a rule of thumb, there is usually around 50 kilometers left when the display lights up.
But you shouldn't push this to the limit. Anyone left with an empty tank will face a fine. While there is no risk of trouble in the event of a breakdown, the situation is different if the vehicle breaks down due to a lack of fuel: it could have been avoided.