They say 15 minutes a day without sunscreen.
5-10 minutes of sun per day is sufficient for the formation of vitamin D. Avoid going out in the sun during the peak hours i.e. from 11 am to 4 pm because the amount of UV rays exposure is maximum during this time. If need arises, apply a sunscreen with a minimum spf of 30 at least half an hour before going out in the sun and wear long sleeved cotton clothes or hats when going out in the sun. Hope it helps. Take care and please do keep me posted on how you are doing or if you have any additional queries.