Since earliest recorded information, man has known of the health benefits the sun gives us. The sunlight treatments were used on everything from coughs and colds and epilepsy; to TB and cancers until the discovery of antibiotics revolutionised medical treatments. When the connection between sunburn and melanoma was made, then sunblock creams and lotions flooded the world, and we had the "cover-up" very successful "slip, slop, slap" program in Australia, and physicians advised people to avoid the sun.
Since then, it has been discovered that healthy sun exposure can be achieved between 10am and 2pm, where a large area of skin needs to be exposed so that the UVB rays can convert vitamin D. This is a problem for a lot of people working a 9-5 job, with few opportunities even with a lunch break to achieve this. Mowing the lawn on the weekend, or going to the beach can often lead to an over exposure of skin to the sun, with an ensuing sunburn which leaves skin vulnerable.
Another interesting discovery is that there appears to be a link to triple-negative breast cancer and low vitamin D levels; in fact, a team of researchers found that Genetic expression was the path that allows tumor cells to grow, and that Vitamin D plays a vital role in turning off that pathway!
So we clearly see then that we all need adequate levels of vitamin D, but if we work during sunlight hours on most days, then we clearly need to get it from somewhere else.
There are UVB tanning beds, or there is supplementation. How much do you take? There are various factors to consider say the experts; how much UVB you do get, your age, weight, and physical activity, and also the reason you are supplementing. Dosages vary from 200 IU daily up to 50,000 IU daily for 6 weeks according to various studies.
It is always safe to see your health practitioner, get your blood levels done, and take his advice on the dosage.
Studies on calcium and Vitamin K 2 have been done in conjunction with Vitamin D as to increasing effectiveness, and various supplements are available commercially.
It is hard to estimate the conditions of the day as to how long you can safely be out in the sun without damage, but I have heard some experts say, that given the various conditions of the day; time, season, cloud conditions, most of us can approximate how long we can be in the sun without burning. They advise to take that estimated time to burn, and only be out in the sun for a third of that time.