> An adult in a bathing suit exposed to 1 minimal
erythemal dose of ultraviolet radiation (a slight pinkness to
the skin 24 h after exposure) was found to be equivalent to
ingesting between 10,000 and 25,000 IU of vitamin D
(Fig. 6).
Doesn't say 30 minutes, but it may be 30 minutes depending on your skin colour and the local strength of the sun.
alt Hacker News
Not OP, but the paper says on page 8
> An adult in a bathing suit exposed to 1 minimal erythemal dose of ultraviolet radiation (a slight pinkness to the skin 24 h after exposure) was found to be equivalent to ingesting between 10,000 and 25,000 IU of vitamin D (Fig. 6).
Doesn't say 30 minutes, but it may be 30 minutes depending on your skin colour and the local strength of the sun.