I think I'm also on the thread starter side where I agree that a short time interval would be good, ranging between 3-5 mins.

Namely because in groups that kills it really quick (i.e. my group takes a min, max of 2 mins), it becomes a matter of who clicks that event page the quickest and runs to the NPC the quickest and run to the summon the quickest, and it actually affects the rankings and rewards of coupons, coins, etc, because no player will actually get to reach Round 10 of the 1st attack, it'll be dead by then.

Also, by having that window, it will be good for more players to take part in it since its not a hunger games style who clicks first, but instead, they can be reminded to enter the field in advance.