The island respawn time appears to be somewhere between 5-14 hours. I checked back about 5 hours after I wiped the island and there were no spawns, but I checked back the next morning ~14 hours later and the zone had respawned.
As of 04/2020 I can confirm that the trick mentioned below about leaving the guardian up works. My first Lord where I wiped the whole island did not drop the Spiroc Wingblade. I came back the next morning and got the Lord to spawn a 2nd time leaving the Guardian up, and he dropped the blade.
On my 2 lvl 85 heroic characters, Cazic Touch never killed me outright, and the most it did was 65k. I brought my cleric just in case, but rezz was not needed.
The whole island was back up the next morning, so I believe Islands 1-6 are on less than ~12 hour respawn. None of the mobs on Island 7 were up after I had cleared them, but was able to spawn the Sister of the Spire on Island 7 by telling Sirran "?llaw eht htiw eno I ma" on Island 6.
Edited, Apr 7th 2020 10:31am by DoctaJay