Some researchers suggest that a few minutes of sunlight each day to the face, neck, hands, and arms are all that is necessary for adequate vitamin D production. However, various factors significantly influence vitamin D synthesis from sun exposure, including age, sun-reactive skin type (Fitzpatrick skin type), geographic location, and season.
For example, at noon in Miami, a person with Fitzpatrick skin type III (darker white skin that burns and tans) would require 6 minutes to synthesize 1000 IU of vitamin D in the summer, but 15 minutes in the winter. A darker skinned individual (e.g. skin type V, brown skin that rarely burns, tans easily) would need longer sun exposure (15 minutes in summer and 29 minutes in winter). In contrast, during a Boston winter, it would take about 1 hour for type III skin and 2 hours for type V skin to synthesize 1000 IU of vitamin D. And that is unlikely to actually happen, as walking around Boston with face, neck and arms exposed for an hour or more in winter is not something that is recommended!