Hello,
After the last day of the event (25/07), Exos, The Blazing (World Boss) will no longer spawn, so this automatically means the achievement & the mount which requires to defeat world boss 3 times will no longer be obtainable in-game, so the quest is also getting removed from everyone.
However, the Event Vendor, Currency, Quest Items, Magmammoth + Daily Quest (the NPC you have to feed for 5 days) will stay available for 2 more weeks to let anyone who is interested to finish their started quests / spend event currency.
After these 2 weeks the event vendor will be removed. As well as the event currency & quests.
So if we go back to your question - if you have unlocked the Magmammoth dailies (defeated World Boss 1 time and completed the first quest) then you will have 2 more weeks time to finish it before it gets removed