It depends! At OmegaQuant we recommend aiming for a Vitamin D blood level >30 ng/mL, but that can be achieved in several ways. Some people may be able to reach this level through exposure to the sun, while some may achieve it through a high-fish diet and fortified foods, and others may take a supplement. Still, most will need a combination of two or three of these. If you go the supplement route, the US Institute of Medicine recommends taking no more than 4000 IU per day on average, but, beyond that, how much supplemental vitamin D you should take depends on your blood level.
The best way to know what you need to do to maintain a desirable blood level of vitamin D is to test regularly and change your habits accordingly. There is a seasonal variability in blood levels depending on your latitude, so your levels could drop during the darkest part of the year and you might want to supplement during that time. On the other hand, during the summer you might get enough through vitamin D through sun exposure and not need any supplemental vitamin D.