I've heard the 15 minutes is all you need. I've also heard that in winter the sun is so weak that no amount of sunshine gives you any. (even if you were naked outside in winter - risking frostbite).
I'm not a medical doctor. I cannot evaluate any of the above claims. I wish I could find a source I could trust.
Depends on where you are. Latitudes above roughly 35 degrees N, the sun is too low in the sky roughly between October and March to allow for UV-B rays to penetrate, which is what your skin needs to synthesize vitamin D.
So yes, if you live in the northern regions, you don't produce any at all from sun exposure, even on a bright sunny day, during most of the year.
Up here in the PNW, even in the summer, you only have a window of roughly 4 to 5 hours where the sun is high enough, in July.
I'm not a medical doctor. I cannot evaluate any of the above claims. I wish I could find a source I could trust.