Kingdom creation is part of the main quest. After forming your kingdom (or joining one of the others), the next quest is to defeat every faction in the game (they surrender when they have lost a certain amount of land). After that you are done with the main quest.
I think the timer stops when you choose a side to support. Problem with that is that the conspiracy timer starts ticking and when that finishes, all empire or non-empire factions become allies and declare war on your faction. So there is still a time limit sort of. Correct me if I'm wrong though since I haven't played the main quest since the change.
For me the AI is currently way worse than Warband. They barely block at all and there are more ways to cheese them compared to Warband.