What the achievement asks
Open ten Forbidden Hoards around the post-storm Forbidden Reach. Ten points, account-wide.
Finding one is not the problem
The Forbidden Hoard is the only treasure on the island that puts itself on your zone map. When one is up you can see it from anywhere on the Reach, so no coordinate list is needed and no detection buff helps.
It spawns guarded. A handful of mobs stand around it and the chest stays sealed until they are dead, which takes a minute on your own and no time at all if anyone else is there.
Spawn points are scattered across the whole island, including inside caves: one regularly appears at 41.16 44.43, underground, where the map icon can look like it is pointing at solid rock.
The clock is the whole achievement
The next Hoard appears roughly thirty minutes after the previous one was first looted, not thirty minutes after it spawned. The timer starts when somebody opens it, so a chest nobody has touched delays the next one indefinitely.
Once opened, it stays lootable for about five minutes, which is generous. There is no daily or weekly limit, so ten Hoards is purely a question of being around for ten spawns.
Thirty-minute intervals mean five hours of wall-clock time at the theoretical best. In practice nobody sits and waits: this is the achievement you leave running in the background while you do the rest of the branch on the same island. Every other Forbidden Reach objective happens within a minute's flight of wherever the next Hoard will be.
Two things worth knowing
Alts count separately for the loot itself, so a second character passing through can open the same Hoard for its contents. The achievement progress is account-wide, but the chest is not one-per-account.
There is a long-standing bug where a realm gets stuck spawning only the War Creche Hoard, sometimes inside a wall. If you have watched three intervals pass with nothing new appearing anywhere, hopping to another realm or another shard fixes it far quicker than waiting does.
Where it fits
One of eleven under You Know How to Reach Me, a piece of A World Awoken. Hoards also drop Zskera Vault Keys, so the five hours here pay directly into Door Buster.