From one who has a serious vitamin D deficiency the official word is:

In summer: 15 mins morning and afternoon with no sun cream on, face and forearms. so half an hour per day total with no sunscreen.
In winter: a total of at least 2.5 - 3 hours a week face and forearms to get adequate vitamin d synthesis.

Vitamin D is fat soluble which means we can store it up so what we get in summer can help to tide us over winter but only if we have lots to start with....

You can get it from foods such as oily fish (salmon or tuna in oil not in brine) and egg yolks but it is only available in very small amounts in foods and is no where near enough for normal levels.

hope that helps Mazzy!