The simple answer to this is that fog lights should be used when visibility is reduced to around 100 meters or less. However there are some other important factors that should be considered when using fog lights on the road.
Firstly, all cars are required by law to have fog lights installed. However, there is no legal obligation for drivers to use them. It might also be the case that a driver simply forgets to put them on, so always be aware.
Some vehicles only have rear fog lights installed, which will help other motorists see that vehicle but the drivers visibility will not be improved.
When fog lights are switched on in a car a fog light indicator will be illuminated on the dashboard of the vehicle. This can be useful for the driver to check if the fog lights are functioning correctly without having to leave the vehicle.