1 full day. And I guess a few hours today checking edits other people made, tweaking parts of the article, responding to comments, etc. Plus, of course, all the background work that went into being able to write it in the first place... ('How long, Mr Whistler?') For example, I spent easily a week researching intelligence GCTAs and measurement error for my embryo selection cost-benefit analysis, which I could mostly copy-paste into that article. (I wanted an accurate GCTA estimate to put an upper bound on how much variance SNPs could ever explain and thus how much gain was possible per embryo. This required meta-analyzing GCTA estimates to get a stable point estimate and then correction for measurement error because a lot of the estimates are using imperfect measurements of intelligence.)

EDIT: and of course, after saying that, I then spent what must have been several other days working on digging up even more citations, improving related articles, and debating heritability and other stuff on Reddit...