I can only say that producing a boss that'd require people to be at it for 18 or more hours shows that there might be a slight continuation from the leveling in said game, since it sounds to me like exactly the same thing: repeat actions A,B and C long enough to obtain a new item/skill/amount of stats for your collection. However when a boss is introduced to get an epic long fight, 18 hours is way too much. Take Kil'Jaeden, Illidan or Archimonde for example (talking WoW here btw). They feel like epic kills not because they take hours (perchance hours of wiping before your first kill, but that's not the point here), but because they are a royal pain in the backside and require you to be working together as an awesome team. It's the teamwork that makes it happen, not the time spent on killing him. Even if Blizzard or Square Enix were to design a final boss to take hours, make it a somewhat accpetable amount along the lines of 10-14 hours, at which point the amount of time needed still excludes most to nearly all people from doing it (which is a good thing I guess), yet is short enough to prevent people from literally getting sick of the fight.
Though I do agree that having to take shifts on a boss is somewhere between really hardcore and really worrying...