It doesnt matter if the timer is 5min or 5h.
If the dungeon is a go stright kill all and kill a boss, then it sucks.No matter who complains from both side if the event is engaging and fun . everyone will dig.

Casuals , the content will be done really fast .
Hardcore , timer dont make the content harder, availability is a good thing for the players, not the developers who have to bring out new thins faster.