In 2010 I made this brief post about the topic, so if you're interested have a look and try calculating your times.
This paper reviews the main factors influencing the synthesis of vitamin D, with particular focus on ultraviolet radiation exposure. On the global level, the main source of vitamin D is the sun. The effect of solar radiation on vitamin D synthesis depends to some extent on the initial vitamin D levels. At moderate to high latitudes, diet becomes an increasingly important source of vitamin D due to decreased solar intensity and cold temperatures, which discourage skin exposure. During the mid-winter season, these factors result in decreased solar radiation exposure, hindering extensively the synthesis of vitamin D in these populations.
Depending on your current Vitamin D status (i.e if your deficient) then these figures may be low. However its a great starting point and for us brits it will be a useful encouragement to get outside for our required sunlight all year round rather than binging when its really hot and sunny.
Calculated Ultraviolet Exposure Levels for a Healthy Vitamin D Status Website