How long do I have to be in the sun to get enough Vitamin D for the day?

How long do I have to be in the sun to get the minimal amount of Vitamin D needed for the day?

Also, does it matter on which part of my body the sun hits?

And finally, if I wear Sun Block lotion to stop the sun from tanning me, will that prevent me from getting the Vitamin D from the sun?

I am 19 and am a male! thank you!