Another thing to understand is DPC didn't really get into creating the Interlude until around mid December last year, and he was still organising the release of the Steam and GOG versions of the game.
So while episode 8 was released around mid November, he worked on bug fixes, the game walkthrough, and the Steam and GOG releases for the next month, while slightly dabbling in the Interlude.
He then took time off for Christmas.
The Steam & GOG versions were released in early January, and he created another Q&A in late January as well. So basically after Episode 8, he spent just shy of two months testing, bug fixing, behind-the-scenes work in preparation for the Steam/GOG release and customer support (and he did find some time to work on the Interlude too). After all that, he then went 100% into developing the Interlude.
By mid January he'd finished most of the writing and half the renders.
Then he got COVID.
With a week or thereabouts he was back at it, and the end of Jan he was done with music, sound effects, and by early Feb all he had left to do was finish animations and polish.
And by mid Feb he was done, with only beta testing and proof reading to be completed. But a major factor of the beta testing was making sure the import stuff worked well. The game was released on March 4.
So if he really started in earnest around early Jan, and released around early March, subtract a week for COVID and a week for the unique testing that would have had to happen for episode 9 if the Interlude didn't exist, all in all, the Interlude took about a month and a half to complete.
If he released it as a shorter prologue like many other episodes have, he may have saved a month. Not really that big a deal.
I think the biggest issue we're seeing with his development is his desire to add more and more extended animations (at least a third of the Interlude development time looks to have been animations), and we're seeing the same trend with episode 9...