Healthy men seem to use 3000–5000 IU cholecalciferol
with 80% of their winter cholecalciferol need coming from the supplies the skin makes from sunshine during the preceding summer months.

Depending on where you live on the globe This calculator will enable you to work out if the sun will provide UVB rays on any particular day of the year However it doesn't take account of Wind Chill factor nor air temperature so while it may be technically possible it is certainly over optimist for the UK for much of the Winter it is simply impracticable to strip off an lie naked.

This shows the chance of getting sunburnt There really is no need at all for anyone to get sunburnt while attempting to raise Cholecalciferol, vitamin d3 status. You only need a quarter of the time it takes for your skin to burn to make sufficient Vit d3. So anyone going pink has been out in the sun for far too long. 20 minutes full body exposure horizontal should be ample.

Those who have a very sensitive skin need to work at improving this. It can be done in several ways.

See these dietary suggestions

The two tablespoons of tomato puree daily certainly increases exposure time by a third.

Vitamin d, omega 3 and green tea also all improve photo protection of skin from ageing.
In vivo relevance for photoprotection by the vitamin D rapid response pathway.

These results are consistent with the proposal that the vitamin D system in skin may be part of an intrinsic protective mechanism against UV damage.

Photoprotective and anti-skin-aging effects of eicosapentaenoic acid in human skin in vivo

Photoprotective effects of green tea polyphenols.

I found I had absolutely no problem with sunburn last year but as I supplemented with Vit d last winter, regularly eat meals with tomato puree, consume omega 3 and linseed meal daily, and drink Green Tea as my favourite drink I wasn't particularly surprised.

Those who are unable to get regular 20minutes midday sun exposure directly on a lot of skin at least 3 times week will need to use supplements. See the Heaney paper above for a sensible level to use.
You need to be aware that often D2 is used in multivit mixes and The Case Against d2 Ergocalciferol is set out here

Anyone concerned about potential toxicity at the above dose levels needs to read The Risk Assessment for vitamin d This summarises all the research on D3 and concludes that up to 10,000iu a day is perfectly safe providing no one continues at this level day after day, week after week, month after month.

As I have made clear the amount you body needs daily is only a third to a half of that amount. You will get a modest amount from oily fish and some countries fortify their milk (UK doesn't) however the amounts from food sources are fairly trivial.

If you are regularly outside in Summer without sunscreen or SPF cosmetics then you probably won't need to supplement at all during the summer, so the question of Months and Months of supplement use should never occur.

The Reports from the Health Research Forum on Sunlight Robbery include more safe sunbathing advice and information on the health benefits this provides.