First of all...there are 2 types of vitamin Ds (which is actually a hormone)
They are Vitamin D2 and Vitamin D3
D2 is found in mushrooms and in animals, while
D3 can only be attained by being out in the sun (no suncreen) or by taking supplements
D2 is dangerous if you overdose, while
it is much more difficult to OD in D3
*I believe D2 is prescription only in the US, although I could be wrong
Secondly...I recently read studies conducted by Harvard, Stanford, and University of California which said by being out in the sun for 30min...w/ minimal clothing though (rough estimate as it is different for everyone, depending on body mass, skin color, and etc.), you receive an intake of ~10,000 IU of D3.
This is generally have no toxic effect on you

That being said, you should do a D3 testing to see how much D3 you should take each day...
But 1000 IU and 2000 IU could be considered the minimum.
But don't go crazy and do the 10,000 IU dose either. It comes in the form of a gel/pill, whose absorption is different from how your body absorbs the vitamin D from the sun...the process if too complicated to explain =/