I don't like the idea of the cooldown at all, and I don't have any idea what exactly its purpose is supposed to be - but since that's not what we're supposed to be commenting on at this point, I'll just say that if we must have a cooldown, 24 hours is definitely better than 72 hours. Anyone left un-allianced for 72 hours is a HUGE risk of being totally wiped out; this is just going to lead to otherwise active players leaving the game in frustration.