Depends on where you live. The sun has to have a direct enough angle for you to be able to make any vitamin D, no matter how sunny it is. Here in Pennsylvania USA, it will take until the end of March before any vitamin D synthesis can happen. Latitude and season play a rote in Australia as well -- and, of course, the sunscreens that many folks use to prevent skin cancer block vitamin D synthesis.
Tergal may want to read "The high prevalence of vitamin D insufficiency across Australian populations is only partly explained by season and latitude." in Environ Health Perspect. 2007 Aug;115(8):1132-9. For Sayre, "Influence of season and latitude on the cutaneous synthesis of vitamin D3: exposure to winter sunlight in Boston and Edmonton will not promote vitamin D3 synthesis in human skin." in J Clin Endocrinol Metab. 1988 Aug;67(2):373-8.

(Can you tell I'm supposed to be giving a lecture on Vitamin D in a few weeks?)