A pooled analysis of randomized controlled trials and prospective studies finds that women with blood levels of vitamin D in the 60 ng/ml range had an 80% lower breast cancer risk compared to women with less than 20 ng/ml.
Another randomized controlled trial found that men given 4,000 IU of vitamin D per day slowed cancer progression and improved tumors in men with low-grade prostate cancer (compared to men given placebo).
In a 2009 paper published in the Annals of Epidemiology by the authors of this study, they recommended vitamin D levels between 40 to 60 ng/ml based on lowest all-cause mortality data.
Many factors regulate how much vitamin D3 a person makes from UVB exposure from the sun. These factors include geographic location/time of year since some northern latitudes do not get UVB exposure certain times of the year, how much pigmentation a person has since melanin acts as a natural sunscreen and blocks UVB radiation, age since the aging process makes the production of vitamin D3 from the sun much less efficient, body fat since vitamin D3 is less bioavailable to mobilize from the skin into the bloodstream with increasing fat mass, and other factors such as sunscreen and time spent in the sun.
The best way to know how much vitamin D to supplement with is to get a blood test before and after supplementation.