my opinion

i think basically , that naturally we would remove ourselves from
direct sunlight after about 20 minutes exposure anyhow,

if we were in tune with our bodies.
we would get hot, and move to a shady area.

even most animals do that (look for shade relief)

so the optimal time is about 15 up to 20 minutes.
which is the time most beneficial to the body.

just a few random thoughts on the subject 8)