#4 - Dec. 21, 2017, 4:38 p.m.
Once all of your older Legendaries are level 1000 you no longer need the quest. Any new ones that drop for you will already be item level 1000 :)
The Wakening Essences will only drop for you while you have the quest in progress though, if you've already upgraded all your Legendaries, hand it in one last time and you won't need it again :)