Does Scarlet's Lair ever go away?
smokes pipe
All things in the world one day end.
looks off thoughtfully into distance
It seems quite likely based on past patterns that it will go away two Tuesdays from now. Or at least that it will when the marionette is gone. There was a dev comment about the marionette remaining for as long as it makes sense for the story, but generally speaking LS content has lasted either 2 or 4 weeks.
Why wouldn’t you use the cores? Their only purpose is to make the key and enter the lair (which you only need to do once on the alt you want to take in there). And assuming you’ve been doing the marionette fights, you’ll have cyphers and those will open chests full of yellows and dragonite (if you’re 80). Plus there’s a plethora of clues to Scarlet’s intentions that are filling this forum with excited speculation.
If you don’t have the cores, just tour around to the thumpers on a few maps and get them. You only need to gather 40, you get 10 for finding the lair.
And as a general rule … it’s best to assume you have one week, and spend the first few days of new content working on it. Unless you don’t mind running out of time. Or at least to assume that the next Monday evening is your deadline for any current must-do’s.
In general it will go away right before you are able to complete it.