How much sun exposure is needed for adequate Vitamin D production?

It has been suggested by some vitamin D researchers that approximately 5–30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually leads to sufficient vitamin D synthesis – length of time should be adjusted for individuals and should be approximately HALF the time it would normally take you to get a sunburn. It is important to note that UV rays can not travel through glass, so indoor exposure is not considered adequate. UV rays can also not travel through dense cloud cover or smog. Sunscreens with a sun protection factor (SPF) of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly; therefore, skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.