How about 15 minutes if sunshine a day??
If done at midday, that would be great.
Don’t burn
Don’t be on meds that caution against sun
If you’re at risk for melanoma, don’t at all.
Expose as much skin as you dare show the neighbors.
Flip like bacon, 10m on each side careful not to double expose :)
Be sure the sun is above 50deg elevation:
https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html
Visit a dermatologist once a year for a look see.
Preferably in the nude. And since many have been blessed with an abundance of extra flab and skin nowadays, the exposure time could easily be cut in half ! ;)