I think if a certain amount of time passes, or if a mod edits it, you no longer can. So if it's an old post, or if someone else edited it, you no longer can. Hope that helps!
editing is admin set... and it could be anything from not allowing you to edit at all to allowing you weeks and weeks. every forum is different. Most leave an hour or so, some 24 hours...
Not a clue. Mods retain the ability to edit, so it was a while after the change before we even knew that a restriction had been introduced. If anyone wants to test it (for science!), then I'd be interested to know the answer.
I'm fairly certain it's 24 hours. I can edit one of my posts from 22 hours ago but nothing earlier than that.