I dug deep for this lol... It looks like somebody ballparked it from a graph, I read it in a news story I can't find. It looks like COVID incidence among study participants comes to a screeching halt somewhere around 8-12 days after first dose

Page 30 of https://www.fda.gov/media/144245/download

See this graph > Cumulative Incidence Curves for the First COVID-19 Occurrence After Dose 1, Dose 1 All-Available Efficacy Population

Also keep in mind this graph is probably based on COVID diagnosis date. Since it's unrealistic to test positive the day you contract COVID, I think it's reasonable to say it might even be effective from day 3-5 on