How long is Irisalis available?

The event runs for 75 days, from September 25th to December 10th, so December 10th is the date to plan around. Irisalis belongs to Season 1, and once that window closes the event is gone with it.

You get Irisalis by completing missions in the Legendary Journey section of the event menu. Those missions run on Irisalis Petals (also called Iris Petals), so the real question for most players is how fast they can collect petals before December 10th.

Where to get Irisalis Petals in time

Open the event menu and press the 'How to obtain' button in the bottom left. It lists every petal source and its location, which beats guessing.

How the petals split between sources is not settled. Many come from ordinary play and stay earned permanently: progress on the event's permanent tab, main story and journey quests, side stories, exploration including Breezy Plains, silver badges, a gather task for collecting items around the world, first clears of Alpha boss battles, and fighting Irisalis at the Faded Rainbow. Branch Nurturing gives three petals for each Branch Nurture level of the main branch, levelled by infusing Lumin Amber. There is also a weekly-limited source, and the stamp shop and event shop carry petals too.

The weekly-limited source is the one that punishes a late start. Every week you skip is a week of that allowance you cannot claw back before December 10th.

Not settled: How much of the petal total comes from permanent progress and how much from the stamp shop, event shop and weekly-limited source is not settled, so how late you can start and still finish the Legendary Journey is an open question.