[worldadventurer]

Use this calculator to figure out how much sun time you need given your latitude/longitude, cloud cover, elevation, etc. https://fastrt.nilu.no/VitD.html

[m0llusk]

And keep in mind that using your skin to generate Vitamin D from the sun is a complex process that takes time. Exposure during the day typically results in Vitamin D being made available in the evening, and thorough washing can reduce yield.