You need 10 to 15 minutes of all-over sun on a 'UV index 7' day.
You will need longer if the UV index is lower (e.g. 20 to 30 minutes if the index is 3.5) and less time in the sun if higher (e.g. 5 to 7.5 minutes if the index is 14).
Staying out longer than that won’t make any more vitamin D. Check the UV index online for your local area and time of day.