There's unfortunately not a lot of concrete info to go off of because teams have incentive to keep these things close to the vest. But generally some things to keep in mind
- Historically the rule of thumb for increasing a young pitcher's innings limit was "your old career high in innings plus 30"
- It was shown this was bunk like a decade ago, but around the league teams continued to mostly follow it. My thought was the that teams had pushed back on the specifics not the general idea
- The Cubs among other teams have said over the last few years that they use technology to monitor for fatigue instead of simply counting innings. I believe them to an extent, though in practice "career high +30" has been stayed a pretty solid rule of thumb
- The Cubs this past winter added pitching guru Tyler Zombro to their front office and also hired a Japanese consulting firm that specializes in pitcher health
- Over the last few years, a bunch of veteran relievers have converted back to starting. For these veterans, they have generally ended up around 150 IP their first year back in the rotation. Regardless of how long they'd been relieving, their previous career high, etc.
- Last year Garrett Crochet coming off of TJ and having never exceeded 70 innings in a year in his life, made 32 starts and threw a shade under 150 innings. The White Sox pulled this off not by messing with Crochet's schedule but by shortening his outings, particularly in the second half. For the year he ended up throwing just under 75 pitches per start. This year Crochet is leading MLB in innings
So take it all together, and my guess is that the current wisdom is that whether young or old more or less any pitcher (at least of MLB age) can be asked to throw at least 150 innings, and the question is just how do you want to finagle things to end up at that target. Shorter outings like Crochet, a fake trip to the IL, shifting to relief, etc.
The big question is the playoffs, because between Crochet and all those converted relievers, I can't find any who pitched in October. This year Jacob Misiorowski (about 15 innings behind Cade) and Clay Holmes (about 15 innings ahead) both will. But the Seth Lugos of the recent past did not and so aren't helpful historical examples on this part. I would suspect that Horton will be allowed to pitch once per series. That would give him plenty of rest between each start. So it would add as much as 20 more innings, but doing so over the course of an extra month probably isn't especially problematic?