Nobody really knows, but the general advice is to get "10-15 minutes of sun every day, when your shadow is shorter than you". The above is probably enough if y

Nobody really knows, but the general advice is to get "10-15 minutes of sun every day, when your shadow is shorter than you". The above is probably enough if you're caucasian, but add a bit of color and you'll need more time. Also, UV-B rays needs to be able to penetrate the atmosphere, which is why the shadow part is there. I live at 56 degress north, and the shadow thing becomes a problem from around September to April. First of all, the sun doesn't rise above 50 degress until late april, and it also needs to be visible, which it usually isn't for long.