This is a difficult question to answer, it depends :-)
Your skin colour plays an important role in establishing how long you should remain in direct sunlight, the darker the skin the longer you will need to be exposed to sunlight.
For example a light skinned person in July in europe would only need 10 minutes in the sun whereas a dark skinned person to achieve the same result would need 45 minutes in the sun.
A good guide is the sun calendar, BUT in the winter months all wrapped up you will find it extremely difficult to boost your levels to what is needed.
In the summer however it's a lot easier because you will inevitably be showing a lot more skin to the sun and wearing lighter clothing and maybe even having a swim!
The vast majority of us in the northern hemisphere are low on vitamin D and a supplement is necessary.
Our lifestyle is not at all conducive to getting a lot of sun, we seem to spend more time in front of a screen than out in the daylight.
There are many variables affecting your vitamin d levels, those variables being YOU, because you are different from me! Your weight, height, metabolic rate etc...the only reliable way is to have a blood test that will determine your vitamin D levels.
But unless you live on or near the equator and spend a lot of time on the beach the likelihood is that you will be deficient in the sun vitamin that is fast becoming more important to maintain our health and our immunity.