We decided to not have IPOs on new game worlds due to both cheating and gameplay reasons. I think there was a bit of explanation about it in the blog entry when Hoover was launched.
The stock exchange is already heavily restricted in the game, yet still poses a lot of issues. Yes, we could tighten some game rules even more and impose technical limitations were possible, but that would make it even less of an actual stock market.
In essence, it's not a properly working feature anymore, yet still causes troubles - so it just didn't seem wise to still introduce a broken feature on new game worlds.
But we do know that it would still be nice to be able to buy and sell airlines or make investments or support other airlines. So instead, the fact that we're not having IPOs anymore, opens up the possibility to have a replacement feature in the future, that incorporates the gameplay options we want without the current drawbacks.
There’s is usually no official time, because launching a new server isn’t an every day thing. Sometimes everything works like a charm, sometimes there’s some issues.
But anywhere between noon and late afternoon is the most common thing, if I remember correctly.