Personally, I've been taking 5000 IUs/day and recently started stepping that closer to 10,000 IUs. I use the liquid drops which are in oil, and I always try to take them with some sort of meal. The liquid is a lot cheaper and seems like the ideal way to absorb it to me.

This is in cloudy wintertime Seattle. In sunny times it may be completely unnecessary.

If I recall correctly, such a treatment is not yet evidence based, though clinical trials are ongoing.