The standard advice — eye level — is not much use, because eye level changes depending on whether you are standing in a hallway or lying in bed. It is more useful to work backwards from what the light is doing.
In short. Bedside — 1.5m · Hallway — 1.6 to 1.8m · Either side of a mirror — 1.7m · Stairs — 1.8m from the tread
Beside a bed
Around 1.5m from the floor, or roughly 600mm above the mattress. High enough to clear a headboard, low enough that the light falls onto a book rather than onto your forehead. If you read sitting up, aim slightly lower and slightly forward of the pillow.
Two matched lights either side is the obvious arrangement, and it is obvious because it works. If the bed is against a chimney breast and there is only room for one, put it on the side of whoever reads.
Along a hallway
Between 1.6 and 1.8m, and higher in a hall with tall ceilings. A hallway light is not there to light the hallway — it is there to give the wall some warmth and stop the space reading as a corridor. Space them 2 to 2.5m apart and accept the pools of light between them. That rhythm is the effect.
Either side of a mirror
About 1.7m, level with the top third of the mirror. Lights either side give even light across a face; a single light above casts shadows down it, which is why bathroom mirrors lit from above are unflattering.
On a staircase
1.8m measured from the tread directly below, not from the floor at the bottom. Otherwise the lights march up the wall at the wrong angle.
Two things worth knowing before you drill
First, measure to the centre of the backplate, not the top of the fitting. Every dimension we publish gives you the backplate size and the fixing centres so you can mark it accurately.
Second, if the wall is going to be plastered or papered after first fix, get the cable in the right place now. Moving a wall light 100mm later is a chase, a patch and a repaint.