Scouts Tavern is available in ch20 and can reduce scout times by 65%. Which chapter do Scouts times start taking 5 days?
It is not so much what chapter you are in as how many provinces you have scouted. I am sitting at the end of chapter 20 in all my cities (waiting until after the FA to start ch 21). In my Arendyll city, I currently have 659 provinces scouted. Scout's Tavern is currently at level 15, which means my scout time is 58% of what it would normally be. Without Scout's Tavern, my scout time would be 98 hours (4 days, 2 hours). With Scout's Tavern it is 57 hours. However, I have only been playing since the start of 2020 and have moved through the game rather fast. Somebody who has been playing longer could easily have many more provinces scouted than I currently have and have a longer scout time.
And it was actually worse in chapter 19. The reason for this is that chapter 20 opens up with an Advanced Scouts tech. This was the first Advanced Scouts tech offered since the start of chapter 15. This stretch of 4 chapters with no Advanced Scouts techs is when scout times really get out of hand. It has been a long time, but I seem to remember my scout times were well over 100 hours in ch 19 before I got the Advanced Scouts tech at the start of ch 20 and received the -25% reduction it gave. Unfortunately chapter 21 does not have an Advanced Scouts either, so my scout times will just continue to grow while I do that chapter.