Well, my goal is to use them during book 3 of Wrath of the Righteous, where the estimated total downtime is roughly 3 months or so. So, no Kingmaker. But long enough to see if we can find some enjoyment with them. And to see if Kingmaker is a possibility for our group at some point.
3 in-game months is, from my limited exposure to Kingmaker's kingdom building rules, and more exhaustive experience with Ultimate Campaigns, is barely sufficient to get the ball rolling on either.
I also would say you're creating a potential conflict with players needing to go do stuff in the world, and at the same time wanting/needing to manage/protect their precious new buildings that they just spent a ton of wealth on they need to recuperate.
If you're doing Ultimate Campaign, and have ANY Math People, Engineers, Spreadsheet Gurus, etc etc... you HAVE to, HAVE to either limit them to a fixed amount of space or limit them to a SINGLE building. UC Downtime Buildings are absolutely broken without some bound in place.