typically between 10am and 2pm, I have read that at certain latitudes and times of year the required window is considerably less but cannot find a reference.
http://www.return2health.net/articles/does-vitamin-d-really-...
The fact is that if you live in higher latitudes and have dark skin, you physically cannot absorb enough vitamin D from sunlight during the winter months. While it is physically possible during the summer, it is not practical to spend hours sunbathing.
The fact is that the majority of people of planet earth can go out in the sunshine at the right time of day and get 80-90% the vitamin D they need and get the rest from a balanced diet.
I said the above viewpoint was caucasian centric because it assumed that everyone could get enough vitamin D from the sun. My response is not 'coloured'-centric because I did not say that no one could get enough from the sun, only that your absorption depends on your skin color. My response was not a generalization, unlike the comment I replied to.
As one of the sibling commenter said, it very much depends on your skin shade and the location on Earth.