15 minutes per day, 3 - 4 times a week exposure to sunlight seems to be the KEY

 The leading natural source of vitamin D is sunlight exposure stimulating synthesis in the skin

Specific summary recommendations include:

• Clinicians should recognize that vitamin D deficiency and insufficiency are widespread, affecting up to 70% of European populations (including those living in sunny regions).
• Healthy postmenopausal women may achieve adequate serum concentrations of vitamin D through either sun exposure (15 minutes per day, 3 - 4 times a week) or supplementation with 800 to 1000 IU/day.
• To achieve adequate levels, women with low serum 25(OH)D may need doses ranging from 4000 to 10,000 IU/day.
• Specific tailored doses of vitamin D supplements are needed for women with morbid obesity, both before and after gastrointestinal bypass surgery, malabsorption syndromes, and/or hepatic or renal diseases.
• Adequate amounts of vitamin D and specific bone-conserving therapies are indicated for women with vitamin D deficiency, osteoporosis, and/or previous incidental fractures. If there are no associated risk factors for low serum vitamin D levels, doses should be from 800 to 1200 IU/day.

source: medscape.com