Not sure I would agree that "most of us need to take a fairly high doseage for several months" to have proper levels.
Whether you are or will become deficient in Vitamin D or not is very much dependent upon your diet, and exposure to sunlight - but it does not need to be strong sunlight, and it does not have to be a large part of your body or a long period of exposure in order for the body to synthesise the Vitamin D that is required. Bare arms in a cloudy sky for around 15 minutes a is probably sufficient, depending on ethnic origin.
It is true to say that there is certainly much review and debate going currently over what should constitute a normal level of Vitamin D in the blood.
The Chief Medical Officers recommendations in the UK are that pregnant women take regular vitamin D supplements. Children 6 months - 5 years, unless feeding on infant formula.People over 65, and people who get very little exposure to the sun ( from cultural clothing etc)
Taking Vitamin D as a routine supplement will not harm you, but it is debateable how much benefit you will see, unless you are in one of the highlighted at risk groups.
http://www.dh.gov.uk/...alasset/dh_132508.pdf
Personally, I think vitamin D the only vitamin supplement I might consider taking, simply because minimum levels are still open to debate, I do not get as much sunshine as I used to, and an excess of vitamin D is not harmful.