At noon, in summer, in Spain. And probably 25% of your skin needs to be hit by direct perpendicular sunlight, and not just be exposed. In any case, don’t assume, go measure your Vitamin D levels.
Sunlight intensity varies greatly. Depending on where you live, season and cloudiness, you may only get a small fraction of the amount implied above. Again, there is no substitute for measuring your levels. Speaking from experience.