They tend to spawn every 4 hours i do believe and also believe there is about 2~4 every zone, dungeon tend to be the easiest to find especially with the lvl limit changed. :P
I don't think it's every 4 hours. I found a chest in CP and then found one right after that, no later than 10 minutes after the first. It's really easy to just go around in fields that you no longer aggro any monsters and look for them, but the loot is generally not so good. Ores, return scrolls, maybe an enhancement stone or pet. At least, at lower-lvl maps.