It is stated in the traffic rules that you must not use full-beam headlights in a way that will blind other road users.

It is, however, necessary to light up the road as much as possible, which is why you should not switch to dipped headlights before there is an actual risk of blinding someone. As a general rule, you should switch to dipped headlights only when the illuminated stretches of road meet. This is usually when the distance to the oncoming vehicle is about 200-300 metres (because headlights project approximately 100 metres ahead).