According to our infos it is during PD12-16
The first hetero "sex scene" is within the first 4 days, so between PD12-16, which I assume is what you're referring to. But that has no guarantee to be with Sophia, since "important character" also includes Dylan or Ellie. So that first hetero scene could easily be Ellie/Logan, or Ellie/Sam, since we now have confirmation Sam will also be appearing on PD16, as I have long suspected.
There is also
this post, which says that the first sex scene with Sophia wont be until PD19, which coincides with the 4th lesbian scene, which leaves the next chance for one of the guys to be PD20, as I said.
I have my own spreadsheet with projections on how long the game will take to complete, from back when PDs were split into thirds, which also happens to track which version numbers will start each PD. PD19 finishes at v.500, so the sex scene on PD20 should happen sometime between v.500 and v.550, which was the question that was asked.
And since my spreadsheet also includes a rough estimate of when that might happen, I included that info too.