I believe it will soon stay longer and spawn faster as the event gets further on. Like with the zombie plague. it got harder to deal with and harder to get rid of.
Had a well thought out post but lost it, I didnt get the buffs doing the rifts when I had dropped my totems, but got it when I didn't drop my totems. If you are having trouble getting the buffs for the achievement, Try doing it without any active class buffs. may help.
I arrived at Hrothgar's Landing, just to see the rift despawn at 22:50, with 3 mobs left to kill. Then later at 23:40 they respawned again, this time I did kill em all. Then I went exploring other zones, and found a fire rift in Zul'Drak at Rageclaw's Den, near the lake. Closed that one too. Then I was in Crystalsong just on time to see the wind rift there despawn at exactly 23:50.Pattern?Anyhow, I'm on the EU realm Nagrand, in case anyone wants to know.PS: Have yet to see any rifts outside these 10 minutes of time.
Is it in specific subzones, or throughout the zone as a whole?