It's difficult to quantify how much since skin pigmentation affects how much radiation your skin absorbs: The darker the skin, the more it's protected against skin cancer but the less able it is to absorb UV-B rays. It also depends on how much skin is exposed and the time of day. If you're fair skinned and sunning yourself outside in a bathing suit at noon, you only need a few minutes without sunscreen. If you're already tan or of Hispanic origin, you need maybe 15 to 20 minutes. Black skin may require six times the sun exposure to make the same vitamin D levels as a very fair-skinned person, but we need more research on this because the studies that have suggested this have been small. We're starting a study later this year to establish how much skin pigmentation, clothing, sunscreen, and seasonal change affect vitamin D levels, so we should know more about all of this in the near future.