There is an older post about this I think, but sorry, couldn't find it. I am curious at what point did you/will you guys stop upgrading your factories? Like do you/did you stop at lvl 27, or you go further to lvl 33 or even further?
Before anyone says how it takes many years to even get to lvl 27, please stop. It depends on your time schedule/priorities in game. On windows it takes me 1,5 year (little less) to upgrade every factory to 27 (trains/islands already at 21). I have 3 more months to go, only 6 factories left and they are already at lvl 15 (except festive, which windows did not get yet -_-.
And that's PC, mobile will be even quicker considering I can play anywhere and we have events like "ore galore" and the first "experiements" which gave us oppurtinities to get a bunch of ores. Without these bonuses I run out of ores using the booster for too long. I do realise you need a lot of time, but you don't actually need money for it. Just Raja.
So lvl 27 is already good enough, but I wonder if someone went further. 33 seems reasonable to me, either + 100% coins or + 100% Xp or 50/50 or maybe something else. I am definetely gonna stop at 27 on windows, but maybe not on mobile.