[pepper22]

I've checked the JWiki and it says that since their certain update the chest spawns every 4 hours.

I can confirm that they do spawn in the same general area as I had a time where I'd go around looking for them after every server check. I've always found the same treasure chest 5-6 times in the same spot. Also depending on the level range of the map the treasure will be better than if you found one in Forest of Mist.

You're likely to find treasure chests in party dungeons but not in caves apparently. You could always go and search for em to be sure but I've yet to have heard of anyone finding one in caves.

Yep, 4hours seems right. Boss mobs spawn every 4 hours as well. I have been farming the Zombie King(korean server) for 3 days now, and every 4 hours there is a Zombie King and a chest(the chest is in a different spot,but within the same room). Good times~~