From MedPageToday June 2012

Others suggested that the levels of vitamin D in studies (400 IUs) are "the equivalent of about 24 seconds of sun in your swimming suit at a Los Angeles beach in the summer. No wonder there is no benefit.
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See also VitaminDWiki

• Overview of Time in the Sun Close!
  4,000 IU in 7 minutes ==> 400 IU in 42 seconds in shorts
  might be 35 seconds in a swimming suit
  3X longer if Dark Skin of elderly