I can tell you from experience that not everything respawns in this game.
I have two level 40+ and one level 30+, and I do a lot of off-road roaming/rangering. I've seen some things that respawned after I returned to an area several days later. But, there are several areas where even weeks later, nothing has respawned.
I don't mind respawning because that gives me a chance to go back in and get more experience points. This works for me because I play in areas where the enemies are little above my character's level, so the points are real good. The fights are awesome, too. Nothing like sneaking through a closed door and and have enemies on the other side. It may take me 4-5 tries to get it right, but eventually I'll kill them all without dying.
Good luck.