Board Thread:Casual Discussion/@comment-31.221.86.178-20160411141003/@comment-26143371-20160411203750

until a quest line kill-date is assigned Which usually means one month after the end of the event.

Everything related to them that rewards you with event currency, comes to an end at most once you update with the takedown update of the event. Sometimes all or at least some quests of the main questline do not reward event currency, see for example Meet My Trophy Life (Act 1), (Act 2) and (Act 3) from the Tap Ball 2015 Event or A Rail of One City, Rail Against the Machine and One Track Mind from the Monorail 2015 Event. Those quests will disappear too. As a more general, but less "clear" rule one could say, everything that tells the storyline of the event will be gone once you do the store-update.

Aside from all this nearly every quest that is unlocked through a building, a character or a costume is permanent. This means there is no end date to those and they can be completed after the end of the event when there would be some idle-time. Those questlines are not related to the event at all, they solely depend on the character/costume/building to be unlocked.

By "nearly" I refer to cases where a character or costume is unlocked as part of the main questline of an event (e.g. the Barbarian and his castle). In such cases there is simply no specific quest related to them.