--the manufacture of vitamin D3 in the skin is subect to a feedback mechanism, which varies from person to person--after a certain amount is made, the skin pigmentation changes and the chemical reaction that produces D3 can no longer advance without a sufficient refractory period (which is many hours in most people). Generally, you know you've reached this state when the skin starts to turn pink.

Ted Hutchinson has written a lot about this process on Braintalk and other places (he's posted here intermittently). Googling "Ted Hutchinson Vitamin D" gets you a lot of material.