Whenever you want to take control of it from the CEO player.
That's part of the game but it's frowned upon because it's a hostile action. CEOs like to keep their corps.
You may want to do this if you feel you can run it better so your country will make more money from it, if you don't want public corporations (because of some ideological belief or whatever), if you are at war with a country who is also a CEO and has a corp in your country, or maybe some other reasons. Just be aware that there could be some sort of backlash. It's basically an unfriendly act, but it's your country and your decision.
You should, out of respect, contact the CEO before doing anything like that.
BC