I need to clarify. No hat, no shirt and preferably in swimsuit for 15 minutes in the summer sun (here in the midwest) to get enough Vitamin D. Winter, Fall and Spring you may need more sun depending on where you live. As we live in the midcoast I can only reduce our Vitamin D in the summer months. The rest of the year even with our sun exposure we need to supplement with Vitamin D3.
Note this articles does say that depending on your skin pigmentation you may need more or less sun.
Half of my family is fair skin and the other half olive skin. So when you are determining how much time you need of sun consider skin tone.
We don't like to sun bathe either that's why we play at the park or play outside to get some Vitamin D. It's so hard for us because skin cancer runs in the family on both sides so I am always balancing sun versus skin cancer.