From what I've read I believe that may be a bit more than what is necessary, but I'm not in the medical field and really know nowhere near enough about it to know if that is anywhere near accurate. I'm sure this information is out there and that if you use a bit of Google-Fu you'll get a good idea of what is required.
Keep in mind also there are other factors besides just time involved here. There is more UV radiation in tropical areas as opposed to closer to the poles, more in the hours between 10 AM - 4 PM than otherwise, the amount of skin exposed will make a difference and cloud cover can affect what gets
through but maybe not as much as we think.