I recently read a reputable article that said that we should be taking 1500 to 2300 IU of Vitamin D each day. I think I read the article on a life extension web site: http://www.lef.org/news/.
Mayo Clinic says that the upper limit for vitamin D is 2000 IU per day. See http://www.mayoclinic.com/health/vitamin-d/AN01864. In Canada, doctors often recommend 1000 IU per day as a potential cancer preventative.
Previously, I was taking 1000 to 1300 IU each day, every day, rain, snow, or shine, 365 days per year.