The gauge only works when the ignition is switched on.
When the needle reaches the reserve zone, the symbol lights up in the instrument cluster display together with the message Please refuel. At this point there are still about 8 litres of fuel left in the tank. This is your reminder to fill up soon.
The tank capacity of your vehicle is given in the Technical data section page 293.
Never run the tank completely dry. If there is an irregular fuel supply, misfiring can occur. This allows unburnt fuel to enter the exhaust system, which could cause overheating and damage the catalytic converter.