10k IU's a day is what it took me. I found that value almost by accident, while starting to follow Stan Efferding's Vertical Diet, which recommends that as a part of a suite of daily vitamins (it is much better than the name belies). IMO anyone who works a desk job should take 10k IU's daily. I fell out of the habit during covid quarantine and my levels (and my mood) were dangerously low when I came out on the other side.