This depends on several factors
- shade surrounding the building
- the size of windows
- the positioning of windows
- the depth of rooms
- reflectivity of outdoor surfaces
- reflectivity of walls floors and ceilings
One of the ways of measuring the natural daylight falling on a particular area in a room is by calculating the daylight factor (DF). This is the ratio, as a percent, of the daylight that reaches a point inside a room compared with what light would be there if there was no roof or building, in other words just an open sky. (It is measured under an overcast sky and is measured at table top height, 850mm, and is averaged over the room).
The measurement takes into account how much direct sky is visible from within the room, how bright the external surfaces are outside the room and how reflective they are within the room. The diagram shows how the light coming in through a window tends to be much more restricted by surrounding shading than is the case with a roof light.
It works out that a DF of lower than 2% looks gloomy and artificial lighting is required. Between 2% and 5% is usually suitable without artificial lighting provided it is not for a working area (such as reading or in a kitchen) and anything above 5% looks quite brightly lit. There are various formulas for calculating this but it usually comes down to the experience of the architect.
If you are very concerned about a particular lighting situation you can ask for a calculation to be done or you can use the BRS Daylight protractor and the Simplified Daylight Tables described here. The brightness of the sky is assumed to be 5000 lux (lumens/sq.m.) and so a DF of 6% requires a minimum level of lighting of about 300 lux for a horizontal working area. Offices are often lit at 500 lux whereas domestic situations may go down to around 50 lux where only a little background lighting is needed.
Roof lights tend to be very efficient in terms of allowing light into a room compared with the same size of window in a wall. They not only have an unimpeded view of the sky but they are more centrally placed over the area they are lighting.