How does respawning work in KoA? It appears to be almost immediate in the "overworld", but what about in dungeons? Do bosses ever respawn?
Bosses don't respawn. From what I can tell it appears that it takes about 1 to 2 amalur days for a full respawn. Some dungeons have special monsters and don't respond for some reason. Overworld seems to be about 1 day and dungeons seem to take a bit longer. You can literally fast travel from one location to a far away location and back and see all of the enemies back in most areas. Remember, as you progress through the game you will find some enemies don't respawn. That is probably because they are quest monsters. Power leveling is best reserved for a reckoning event with a exp potion added. When doing reckoning mode try to kill as many enemies as you can and then perform a fateshift on an orange enemy with a exp potion already stacked as the exp bonus from the button mashing will apply to all kills while in reckoning mode and then the exp bonus potion will stack as well. When you're fast traveling somewhat regularly, the respawn rates can become quite a nuisance.