I've found that for me, I lose interest in a factory game the moment I have to scale up. It's when progressing through the game means copying an existing solution to a problem and pasting it (a bunch of times) somewhere else rather than coming up with new solutions.
In general, I get bored when I've figured out the solution to a problem but it takes a long time to get it implemented. Which is also why building conveyor belts is typically my least favourite part of any factory game and why I very rarely refactor my base.