Householmes -
Vitamin D capsules are a supplement to the vitamin D your body naturally generates from exposure to sunlight. If you are getting enough naturally, there is no need to take any capsules unless you just want some insurance (there is no downside to taking too much that I'm aware of).
The guideline is around 20 minutes a day of direct sunshine (being next to a window doesn't work unfortunately). If you go for a walk during your lunch break (like I usually do), you're probably fine without any. I'm sure you've read the Sisson stuff so I'll just give you Robb Wolf's guideliness - 2,000-5,000 IU/day, ideally taken in the morning with a meal containing fat. Although he notes some significant factors that affect how much you might be producing naturally - air pollution diminishes it, as does darker skin pigmentation.