The fuel gauge only works when the ignition is switched on. When the fuel gauge reaches the reserve zone, the indicator lamp Link lights up.
The tank capacity of your Audi Q3 is given in the Technical data section Link.
Caution!
Never run the tank completely dry. If there is an irregular fuel supply, misfiring can occur. Unburnt fuel can then enter the exhaust system. This can lead to overheating and damage to the catalytic converter.