Vitamin D is the only dietary nutrient that the body makes in response to direct sunlight and as such, if sun exposure is limited then we need to ingest it and since food sources are limited supplements are a good option. Vitamin D is most well-known for enhancing bone health, but it has been shown to aid in the absorption of minerals and protect against cancer, infections, and many diseases. Vitamin D regulates around 3,000 genes which might explain its many benefits. But how much vitamin D should you get? The Institute of Medicine (IOM) recommends 600IU a day to prevent a deficiency in those with limited sunlight, but this will not even get you close to optimal levels for healthy living.
For healthy adults, Intaking around 5000IU a day is adequate to raise vitamin D to more optimum levels, and intaking up to 10,000IU a day can increase it even more. A more optimal level of Vitamin D in the blood is 50-70ng/ml, which can be achieved for some by intaking 5000IU every day with limited sun exposure (or by being in the sun all day). Intaking up to 10,000 IU can be advantageous for those with little sunlight exposure, and even 50,000IU can help while sick. Toxicity is difficult to achieve as cases are typically from people taking 1,000,000IU or more for more than a month, so toxicity is typically not an issue.
As for sun exposure, more vitamin D is produced in lighter-skinned people and in warmer/clearer weather where the sun is the highest. So if it's cloudy and cold the need for supplementing with vitamin D becomes more apparent. If it is the evening in the winter, it becomes very difficult for someone with fair skin to synthesize enough vitamin D to even be at sufficient levels. But that same person at noon in the summer could synthesize plenty of vitamin D in less than 30 minutes. With colder weather here it becomes important to ensure vitamin D levels with supplementation.