Depending on your skin tone 15 min a day in the sun will give you a full day's supply. Milk also works, lol.
Amount of time depends on your genetic background and geographic location and time of year(european skin for example is fair because it needs to let more UV in, african skin is dark because it needs to keep more out because there is more UV penetration in sunlight near the equator and less farther away in europe).
Research has shown that most north americans are vit d deficient because we don't get enough sun and in the winter most the usa has insufficient UVB in the sun to correct the problem even if we bask.
Milk is for babies. Eat more leafy vegetables.