Depending on your age and color of your skin, choose the length of time with an exposed body part to maximize daily Vitamin D production. Some studies say short exposure each day over the hottest days in the summer will produce enough vitamin D for the whole year, but in the winter there is always cod liver oil or vitamin D supplementation.
Determine the length of time needed based on the following estimates:
the lightest: newborn 5 minutes, child 10 minutes, teen 15 minutes, adult 20 minutes
the darkest: 30 minutes for newborn, 1 hour for a child, 1.5 hours for a teen, adult 2 hours
natural oils on your skin are needed for the conversion of cholesterol to vitamin D so leave skin unwashed for 20 minutes before and after sun exposure
Rotate the exposed part each day maximum for skin cancer prevention.