I'm not a doctor. I'd say long enough to maintain a healthy full-body tan. Skin tone is a mechanism that the body uses to prevent excess D production. Now it's likely that nobody is going to do this [I won't], which means we all need some degree of supplement in or with our diets. For osteoporosis? Remember that D pulls calcium into the blood, but it also pulls it out of the bone. You want both D and something to put that calcium where you want it (vit K2?).