As other comments say, duration of exposure depends on time of year, distance from equator, level of skin pigmentation, etc. One other factor I didn't see ment

As other comments say, duration of exposure depends on time of year, distance from equator, level of skin pigmentation, etc. One other factor I didn't see mentioned (or I just missed it) is amount of skin area exposed to sunlight. Generally it's recommended to protect sensitive skin of face/neck. Exposed limbs, trunk are somewhat less vulnerable to sunlight injury (but varies with skin type). When sunlight has little penetration of layers of clothing it obviously doesn't cause Vit D synthesis in skin. In northern US/Canada, European countries, vit D supplementation is more reliable, possibly safer re: cancer risk. However a lot about Vit D remains controversial. It's still unclear what the optimum target range is (as measure by blood level), and just how much Vit D is too much. As it is now, lots of expert opinions but no consensus in in sight. Finally Vit D accumulates/declines gradually. High doses (50k units) are used in deficiency states, normalization of blood level typically takes a few months. Well worth doing for sake of bone health, and other body systems too. In Vit D insufficiency (low but not as low as deficiency) OTC Vit D is probably enough, but people should ask their healthcare providers about what form and dose are best.