My personal thought is that while they seem to be random, but I think there is still some sort of calculating factor (such as how many chests, regardless of type, gets filled after the previous one collected) into how quickly they can show up.
My play style is:
- Clear Normal Monster Chests 1 to 2 times daily.
- Fight and Fill the Titan Chests Daily (there will be days when this would take longer since my alliance cannot finish off Titans above our current alliance’s capabilities)
- I don’t Raid nor do I revenge, so the Raid Chest sits there unfilled for a very long time. Only filled when I am about to advance on a world map province and the game forces me to raid.
As a side note, they can appear to take the place of any of the chests, it seems. I’ve just had a Holy Chest the other day took the spot normally occupied by the Titan Chest (I have just finished collecting the Titan Chest that day). So just for a short while, I had a Holy Chest (150) and a normal Monster Chest (100) to fill at the same time. Which is not a big deal because monsters you defeat fills both chests, and chances are, you’d be done with Elemental chest before you have to add 1 to the Titan chest.
My last appearance vs. the one I just had seem to be anywhere from 2 weeks to a month, sometimes a little longer. Which, if the number of chests collected is a qualification factor, I guess mine is largely hampered by my non-raiding play style.