Originally Posted by
_C-a-e-r-i-t-h_
A mission or event with clearly defined limits is more enjoyable than one with uncertain limits. Events that are exactly 30, 60, or X minutes are less draining than events that are "60 minutes unless they're 2 hours or maybe 4 hours depending how it goes." Remember Dynamis and Limbus? Those got old fast.
Small extensions are fine, like an added 5 minutes to loot the place after the boss dies. Large extensions are terrible: either make the event large to begin with, or make it have multiple stages that can be unlocked through progression (see also: Einherjar), or something, but whatever the case it is always preferable to know how long you'll be in something before you begin.