Originally Posted by
Freyalise
Simple answer is that it doesn't matter. Consider simple example:
Level A is 3m xp long and you get 1m xp/hour normally. It takes 3 hours normally. If you pop quests it takes 2 hours at 1.5m xp/hour instead
Level B is 30m xp long and you get 10m xp/hour normally. It takes 3 hours normally. If you pop quests it takes 2 hours at 15m xp/hour instead
So the fact that you save more experience if you use it later is a total red herring, since you don't save more time. It doesn't matter even if the later zone also takes longer, each quest saves you 30min levelling time whenever you use it if you assume constant xp/hour throughout any given level.
The only way you can get any 'extra' advantage is by using it so that you are able to completely skip some area which is unusually slow compared to the previous area. So for example if the first bit of Deepholm goes at 2.5m xp/hour, then there is a slow bit of Deepholm which goes at only 2m xp/hour, before Uldum which goes at 3m xp/hour, you should certainly use the quest to skip the slow bit entirely or as much as possible (since with the quest you might ding 83 without it and thus not need it at all). That is only an example (although I hate deepholm anyway).
My plan is to simply use the level 20 quest and daily immediately (in fact ding 81 from discovering kun'lai summit), and then top up just before the buff ends with another two quests. That way you get the 50% xp bonus from the quest *on the next 50% xp quest itself*, instead of waiting for it to run out and not getting it.