From the vitamindcouncil.org:

The amount of vitamin D you get from exposing your bare skin to the sun depends on:

The time of day – your skin produces more vitamin D if you expose it during the middle of the day. Where you live – the closer to the equator you live, the easier it is for you to produce vitamin D from sunlight all year round. The color of your skin – pale skins make vitamin D more quickly than darker skins. The amount of skin you expose – the more skin your expose the more vitamin D your body will produce.